Revert of Switch to use SequencedTaskChecker instead of ThreadChecker where needed. (patchset #1 id:1 of https://codereview.webrtc.org/2149553002/ )

Reason for revert:
Breaks Chrome FYI

https://build.chromium.org/p/chromium.webrtc.fyi/builders/Linux%20Builder/builds/8187/steps/compile/logs/stdio

Original issue's description:
> Switch to use SequencedTaskChecker instead of ThreadChecker where needed.
> This cl is in preparation for https://codereview.webrtc.org/2060403002/ Add task queue to Call.
> In the coming cl the video_sender, and i420_buffer_pool will be used on a task queue and therefore SequencedTaskChecker is needed instead of a ThreadChecker.
>
> BUG=webrtc:5687
>
> Committed: https://crrev.com/ec7cef854d85be2d9fd5b1a4e2c59a67feb662eb
> Cr-Commit-Position: refs/heads/master@{#13474}

TBR=tommi@webrtc.org
# Skipping CQ checks because original CL landed less than 1 days ago.
NOPRESUBMIT=true
NOTREECHECKS=true
NOTRY=true
BUG=webrtc:5687

Review-Url: https://codereview.webrtc.org/2150843003
Cr-Commit-Position: refs/heads/master@{#13475}
diff --git a/webrtc/modules/video_coding/video_coding.gypi b/webrtc/modules/video_coding/video_coding.gypi
index 0b682c7..602f368 100644
--- a/webrtc/modules/video_coding/video_coding.gypi
+++ b/webrtc/modules/video_coding/video_coding.gypi
@@ -14,7 +14,6 @@
       'dependencies': [
         'webrtc_h264',
         'webrtc_i420',
-        '../base/base.gyp:rtc_task_queue',
         '<(webrtc_root)/common_video/common_video.gyp:common_video',
         '<(webrtc_root)/modules/video_coding/utility/video_coding_utility.gyp:video_coding_utility',
         '<(webrtc_root)/system_wrappers/system_wrappers.gyp:system_wrappers',
diff --git a/webrtc/modules/video_coding/video_coding_impl.h b/webrtc/modules/video_coding/video_coding_impl.h
index 40bccf2..6bafd5a 100644
--- a/webrtc/modules/video_coding/video_coding_impl.h
+++ b/webrtc/modules/video_coding/video_coding_impl.h
@@ -19,7 +19,7 @@
 
 #include "webrtc/base/onetimeevent.h"
 #include "webrtc/base/thread_annotations.h"
-#include "webrtc/base/sequenced_task_checker.h"
+#include "webrtc/base/thread_checker.h"
 #include "webrtc/common_video/include/frame_callback.h"
 #include "webrtc/modules/video_coding/codec_database.h"
 #include "webrtc/modules/video_coding/frame_buffer.h"
@@ -108,7 +108,8 @@
 
   // Must be accessed on the construction thread of VideoSender.
   VideoCodec current_codec_;
-  rtc::SequencedTaskChecker sequenced_checker_;
+  rtc::ThreadChecker main_thread_;
+
 
   rtc::CriticalSection params_crit_;
   EncoderParameters encoder_params_ GUARDED_BY(params_crit_);
diff --git a/webrtc/modules/video_coding/video_sender.cc b/webrtc/modules/video_coding/video_sender.cc
index f3e2bfe..aecc60c 100644
--- a/webrtc/modules/video_coding/video_sender.cc
+++ b/webrtc/modules/video_coding/video_sender.cc
@@ -43,7 +43,7 @@
   // Allow VideoSender to be created on one thread but used on another, post
   // construction. This is currently how this class is being used by at least
   // one external project (diffractor).
-  sequenced_checker_.Detach();
+  main_thread_.DetachFromThread();
 }
 
 VideoSender::~VideoSender() {}
@@ -82,7 +82,7 @@
 int32_t VideoSender::RegisterSendCodec(const VideoCodec* sendCodec,
                                        uint32_t numberOfCores,
                                        uint32_t maxPayloadSize) {
-  RTC_DCHECK(sequenced_checker_.CalledSequentially());
+  RTC_DCHECK(main_thread_.CalledOnValidThread());
   rtc::CritScope lock(&encoder_crit_);
   if (sendCodec == nullptr) {
     return VCM_PARAMETER_ERROR;
@@ -150,7 +150,7 @@
 void VideoSender::RegisterExternalEncoder(VideoEncoder* externalEncoder,
                                           uint8_t payloadType,
                                           bool internalSource /*= false*/) {
-  RTC_DCHECK(sequenced_checker_.CalledSequentially());
+  RTC_DCHECK(main_thread_.CalledOnValidThread());
 
   rtc::CritScope lock(&encoder_crit_);
 
@@ -172,7 +172,7 @@
 
 // Get encode bitrate
 int VideoSender::Bitrate(unsigned int* bitrate) const {
-  RTC_DCHECK(sequenced_checker_.CalledSequentially());
+  RTC_DCHECK(main_thread_.CalledOnValidThread());
   // Since we're running on the thread that's the only thread known to modify
   // the value of _encoder, we don't need to grab the lock here.
 
@@ -184,7 +184,7 @@
 
 // Get encode frame rate
 int VideoSender::FrameRate(unsigned int* framerate) const {
-  RTC_DCHECK(sequenced_checker_.CalledSequentially());
+  RTC_DCHECK(main_thread_.CalledOnValidThread());
   // Since we're running on the thread that's the only thread known to modify
   // the value of _encoder, we don't need to grab the lock here.