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}

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 @@ class VideoSender : public Module {
 
   // 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_);