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.h b/webrtc/modules/video_coding/codec_database.h
index 5f8656d..1317e22 100644
--- a/webrtc/modules/video_coding/codec_database.h
+++ b/webrtc/modules/video_coding/codec_database.h
@@ -107,9 +107,9 @@
       const VCMEncodedFrame& frame,
       VCMDecodedFrameCallback* decoded_frame_callback);
 
-  // Returns the current decoder (i.e. the same value as was last returned from
-  // GetDecoder();
-  VCMGenericDecoder* GetCurrentDecoder();
+  // Deletes the memory of the decoder instance |decoder|. Used to delete
+  // deep copies returned by CreateDecoderCopy().
+  void ReleaseDecoder(VCMGenericDecoder* decoder) const;
 
   // Returns true if the currently active decoder prefer to decode frames late.
   // That means that frames must be decoded near the render times stamp.
@@ -121,9 +121,8 @@
   typedef std::map<uint8_t, VCMDecoderMapItem*> DecoderMap;
   typedef std::map<uint8_t, VCMExtDecoderMapItem*> ExternalDecoderMap;
 
-  std::unique_ptr<VCMGenericDecoder> CreateAndInitDecoder(
-      const VCMEncodedFrame& frame,
-      VideoCodec* new_codec) const;
+  VCMGenericDecoder* CreateAndInitDecoder(const VCMEncodedFrame& frame,
+                                          VideoCodec* new_codec) const;
 
   // Determines whether a new codec has to be created or not.
   // Checks every setting apart from maxFramerate and startBitrate.
@@ -131,6 +130,9 @@
 
   void DeleteEncoder();
 
+  // Create an internal Decoder given a codec type
+  VCMGenericDecoder* CreateDecoder(VideoCodecType type) const;
+
   const VCMDecoderMapItem* FindDecoderItem(uint8_t payload_type) const;
 
   const VCMExtDecoderMapItem* FindExternalDecoderItem(
@@ -147,7 +149,7 @@
   bool internal_source_;
   VCMEncodedFrameCallback* const encoded_frame_callback_;
   std::unique_ptr<VCMGenericEncoder> ptr_encoder_;
-  std::unique_ptr<VCMGenericDecoder> ptr_decoder_;
+  VCMGenericDecoder* ptr_decoder_;
   DecoderMap dec_map_;
   ExternalDecoderMap dec_external_map_;
 };  // VCMCodecDataBase