Reland of Change VideoTrack implementation to invoke VideoTrackSourceInterface::AddOrUpdateSink on …

webrtc/pc/videotrack.h

Index: webrtc/pc/videotrack.h
diff --git a/webrtc/pc/videotrack.h b/webrtc/pc/videotrack.h
index f251797155c5b8a2893119530dbb1b2019a48ba0..6e20e978b9b2edcc3e4e1fd4e1c703593e3e3cc9 100644
--- a/webrtc/pc/videotrack.h
+++ b/webrtc/pc/videotrack.h
@@ -27,7 +27,8 @@ class VideoTrack : public MediaStreamTrack<VideoTrackInterface>,
  public:
   static rtc::scoped_refptr<VideoTrack> Create(
       const std::string& label,
-      VideoTrackSourceInterface* source);
+      VideoTrackSourceInterface* source,
+      rtc::Thread* worker_thread);
 
   void AddOrUpdateSink(rtc::VideoSinkInterface<VideoFrame>* sink,
                        const rtc::VideoSinkWants& wants) override;
@@ -42,15 +43,17 @@ class VideoTrack : public MediaStreamTrack<VideoTrackInterface>,
   std::string kind() const override;
 
  protected:
-  VideoTrack(const std::string& id, VideoTrackSourceInterface* video_source);
+  VideoTrack(const std::string& id,
+             VideoTrackSourceInterface* video_source,
+             rtc::Thread* worker_thread);
   ~VideoTrack();
 
  private:
   // Implements ObserverInterface. Observes |video_source_| state.
   void OnChanged() override;
 
+  rtc::Thread* const worker_thread_;
   rtc::ThreadChecker signaling_thread_checker_;
-  rtc::ThreadChecker worker_thread_checker_;
   rtc::scoped_refptr<VideoTrackSourceInterface> video_source_;
   ContentHint content_hint_ GUARDED_BY(signaling_thread_checker_);
 };