Revert of Deliver video frames on Android, on the decode thread. (patchset #7 id:120001 of https://codereview.webrtc.org/2764573002/ )

Reason for revert:
Breaks Chrome FYI Android bots.

See:
https://build.chromium.org/p/chromium.webrtc.fyi/builders/Android%20Tests%20%28dbg%29%20%28L%20Nexus9%29/builds/20418
https://build.chromium.org/p/chromium.webrtc.fyi/builders/Android%20Tests%20%28dbg%29%20%28L%20Nexus6%29/builds/14724
https://build.chromium.org/p/chromium.webrtc.fyi/builders/Android%20Tests%20%28dbg%29%20%28L%20Nexus5%29/builds/20133
https://build.chromium.org/p/chromium.webrtc.fyi/builders/Android%20Tests%20%28dbg%29%20%28K%20Nexus5%29/builds/15111

Original issue's description:
> Deliver video frames on Android, on the decode thread.
>
> VideoCoding
> * Adding a method for polling for frames on Android only until the capture implementation takes care of this (longer term plan).
>
> CodecDatabase
> * Add an accessor for the current decoder
> * Use std::unique_ptr<> for ownership.
> * Remove "Release()" and "ReleaseDecoder()". Instead just delete.
> * Remove |friend| relationship between CodecDatabase and VCMGenericDecoder.
>
> VCMDecodedFrameCallback
> * DCHECKs for thread correctness.
> * Remove |lock_| now that a threading model has been established and verified.
>
> VCMGenericDecoder
> * All methods now have thread checks.
> * Variable access associated with thread checkers.
>
> VideoReceiver
> * Added two notification methods, DecoderThreadStarting() and DecoderThreadStopped()
>   * Allows us to establish a period when the decoder thread is not running and it is safe to modify variables such as callbacks, that are only read when the decoder thread is running.
>   * Allows us to DCHECK thread guarantees.
>   * Allows synchronizing callbacks from the module process thread and have them only active while the decoder thread is running.
>   * The above, allows us to establish two modes for the thread, single-threaded-mutable and multi-threaded-const.
>   * Using that knowledge, we can remove |receive_crit_| as well as locking for a number of member variables.
>
> MediaCodecVideoDecoder
> * Removed frame polling code from this class, since this is now done from the root thread function in VideoReceiveStream.
>
> VideoReceiveStream
> * On Android: Polls for decoded frames every 10ms (same interval as previously in MediaCodecVideoDecoder)
> * [Un]Registers the |video_receiver_| with the module thread only around the time the decoder thread is started/stopped.
> * Notifies the receiver of start/stop events of the decoder thread.
> * Changed the decoder thread to use the new PlatformThread callback type.
>
> BUG=webrtc:7361, 695438
>
> Review-Url: https://codereview.webrtc.org/2764573002
> Cr-Commit-Position: refs/heads/master@{#17527}
> Committed: https://chromium.googlesource.com/external/webrtc/+/e3aa88bbd5accadec73fa7e38584dfbf6aabe8a9

TBR=sakal@webrtc.org,mflodman@webrtc.org,stefan@webrtc.org,tommi@webrtc.org
# Skipping CQ checks because original CL landed less than 1 days ago.
NOPRESUBMIT=true
NOTREECHECKS=true
NOTRY=true
BUG=webrtc:7361, 695438

Review-Url: https://codereview.webrtc.org/2792033003
Cr-Commit-Position: refs/heads/master@{#17530}
diff --git a/webrtc/modules/video_coding/codec_database.cc b/webrtc/modules/video_coding/codec_database.cc
index 29899fc..4d5e202 100644
--- a/webrtc/modules/video_coding/codec_database.cc
+++ b/webrtc/modules/video_coding/codec_database.cc
@@ -71,31 +71,6 @@
   return h264_settings;
 }
 
-// Create an internal Decoder given a codec type
-static std::unique_ptr<VCMGenericDecoder> CreateDecoder(VideoCodecType type) {
-  switch (type) {
-    case kVideoCodecVP8:
-      return std::unique_ptr<VCMGenericDecoder>(
-          new VCMGenericDecoder(VP8Decoder::Create()));
-    case kVideoCodecVP9:
-      return std::unique_ptr<VCMGenericDecoder>(
-          new VCMGenericDecoder(VP9Decoder::Create()));
-    case kVideoCodecI420:
-      return std::unique_ptr<VCMGenericDecoder>(
-          new VCMGenericDecoder(new I420Decoder()));
-    case kVideoCodecH264:
-      if (H264Decoder::IsSupported()) {
-        return std::unique_ptr<VCMGenericDecoder>(
-            new VCMGenericDecoder(H264Decoder::Create()));
-      }
-      break;
-    default:
-      break;
-  }
-  LOG(LS_WARNING) << "No internal decoder of this type exists.";
-  return std::unique_ptr<VCMGenericDecoder>();
-}
-
 VCMDecoderMapItem::VCMDecoderMapItem(VideoCodec* settings,
                                      int number_of_cores,
                                      bool require_key_frame)
@@ -123,12 +98,13 @@
       external_encoder_(nullptr),
       internal_source_(false),
       encoded_frame_callback_(encoded_frame_callback),
+      ptr_decoder_(nullptr),
       dec_map_(),
       dec_external_map_() {}
 
 VCMCodecDataBase::~VCMCodecDataBase() {
   DeleteEncoder();
-  ptr_decoder_.reset();
+  ReleaseDecoder(ptr_decoder_);
   for (auto& kv : dec_map_)
     delete kv.second;
   for (auto& kv : dec_external_map_)
@@ -415,10 +391,11 @@
   // We can't use payload_type to check if the decoder is currently in use,
   // because payload type may be out of date (e.g. before we decode the first
   // frame after RegisterReceiveCodec)
-  if (ptr_decoder_ &&
-      ptr_decoder_->IsSameDecoder((*it).second->external_decoder_instance)) {
+  if (ptr_decoder_ != nullptr &&
+      ptr_decoder_->_decoder == (*it).second->external_decoder_instance) {
     // Release it if it was registered and in use.
-    ptr_decoder_.reset();
+    ReleaseDecoder(ptr_decoder_);
+    ptr_decoder_ = nullptr;
   }
   DeregisterReceiveCodec(payload_type);
   delete it->second;
@@ -478,11 +455,12 @@
   RTC_DCHECK(decoded_frame_callback->UserReceiveCallback());
   uint8_t payload_type = frame.PayloadType();
   if (payload_type == receive_codec_.plType || payload_type == 0) {
-    return ptr_decoder_.get();
+    return ptr_decoder_;
   }
   // Check for exisitng decoder, if exists - delete.
   if (ptr_decoder_) {
-    ptr_decoder_.reset();
+    ReleaseDecoder(ptr_decoder_);
+    ptr_decoder_ = nullptr;
     memset(&receive_codec_, 0, sizeof(VideoCodec));
   }
   ptr_decoder_ = CreateAndInitDecoder(frame, &receive_codec_);
@@ -493,26 +471,36 @@
   callback->OnIncomingPayloadType(receive_codec_.plType);
   if (ptr_decoder_->RegisterDecodeCompleteCallback(decoded_frame_callback) <
       0) {
-    ptr_decoder_.reset();
+    ReleaseDecoder(ptr_decoder_);
+    ptr_decoder_ = nullptr;
     memset(&receive_codec_, 0, sizeof(VideoCodec));
     return nullptr;
   }
-  return ptr_decoder_.get();
+  return ptr_decoder_;
 }
 
-VCMGenericDecoder* VCMCodecDataBase::GetCurrentDecoder() {
-  return ptr_decoder_.get();
+void VCMCodecDataBase::ReleaseDecoder(VCMGenericDecoder* decoder) const {
+  if (decoder) {
+    RTC_DCHECK(decoder->_decoder);
+    decoder->Release();
+    if (!decoder->External()) {
+      delete decoder->_decoder;
+    }
+    delete decoder;
+  }
 }
 
 bool VCMCodecDataBase::PrefersLateDecoding() const {
-  return ptr_decoder_ ? ptr_decoder_->PrefersLateDecoding() : true;
+  if (!ptr_decoder_)
+    return true;
+  return ptr_decoder_->PrefersLateDecoding();
 }
 
 bool VCMCodecDataBase::MatchesCurrentResolution(int width, int height) const {
   return send_codec_.width == width && send_codec_.height == height;
 }
 
-std::unique_ptr<VCMGenericDecoder> VCMCodecDataBase::CreateAndInitDecoder(
+VCMGenericDecoder* VCMCodecDataBase::CreateAndInitDecoder(
     const VCMEncodedFrame& frame,
     VideoCodec* new_codec) const {
   uint8_t payload_type = frame.PayloadType();
@@ -525,13 +513,13 @@
                   << static_cast<int>(payload_type);
     return nullptr;
   }
-  std::unique_ptr<VCMGenericDecoder> ptr_decoder;
+  VCMGenericDecoder* ptr_decoder = nullptr;
   const VCMExtDecoderMapItem* external_dec_item =
       FindExternalDecoderItem(payload_type);
   if (external_dec_item) {
     // External codec.
-    ptr_decoder.reset(new VCMGenericDecoder(
-        external_dec_item->external_decoder_instance, true));
+    ptr_decoder = new VCMGenericDecoder(
+        external_dec_item->external_decoder_instance, true);
   } else {
     // Create decoder.
     ptr_decoder = CreateDecoder(decoder_item->settings->codecType);
@@ -550,6 +538,7 @@
   }
   if (ptr_decoder->InitDecode(decoder_item->settings.get(),
                               decoder_item->number_of_cores) < 0) {
+    ReleaseDecoder(ptr_decoder);
     return nullptr;
   }
   memcpy(new_codec, decoder_item->settings.get(), sizeof(VideoCodec));
@@ -563,6 +552,26 @@
   ptr_encoder_.reset();
 }
 
+VCMGenericDecoder* VCMCodecDataBase::CreateDecoder(VideoCodecType type) const {
+  switch (type) {
+    case kVideoCodecVP8:
+      return new VCMGenericDecoder(VP8Decoder::Create());
+    case kVideoCodecVP9:
+      return new VCMGenericDecoder(VP9Decoder::Create());
+    case kVideoCodecI420:
+      return new VCMGenericDecoder(new I420Decoder());
+    case kVideoCodecH264:
+      if (H264Decoder::IsSupported()) {
+        return new VCMGenericDecoder(H264Decoder::Create());
+      }
+      break;
+    default:
+      break;
+  }
+  LOG(LS_WARNING) << "No internal decoder of this type exists.";
+  return nullptr;
+}
+
 const VCMDecoderMapItem* VCMCodecDataBase::FindDecoderItem(
     uint8_t payload_type) const {
   DecoderMap::const_iterator it = dec_map_.find(payload_type);