Change VideoTrack implementation to invoke VideoTrackSourceInterface::AddOrUpdateSink on the worker thread.

Added documentation of thread expectations for video tracks and sources to the API.

BUG=None

Review-Url: https://codereview.webrtc.org/2964863002
Cr-Commit-Position: refs/heads/master@{#18938}
diff --git a/webrtc/api/mediastreaminterface.h b/webrtc/api/mediastreaminterface.h
index cc660e1..71cbbb7 100644
--- a/webrtc/api/mediastreaminterface.h
+++ b/webrtc/api/mediastreaminterface.h
@@ -111,6 +111,11 @@
 
 // VideoTrackSourceInterface is a reference counted source used for
 // VideoTracks. The same source can be used by multiple VideoTracks.
+// VideoTrackSourceInterface is designed to be invoked on the signaling thread
+// except for rtc::VideoSourceInterface<VideoFrame> methods that will be invoked
+// on the worker thread via a VideoTrack. A custom implementation of a source
+// can inherit AdaptedVideoTrackSource instead of directly implementing this
+// interface.
 class VideoTrackSourceInterface
     : public MediaSourceInterface,
       public rtc::VideoSourceInterface<VideoFrame> {
@@ -145,6 +150,12 @@
   virtual ~VideoTrackSourceInterface() {}
 };
 
+// VideoTrackInterface is designed to be invoked on the signaling thread except
+// for rtc::VideoSourceInterface<VideoFrame> methods that must be invoked
+// on the worker thread.
+// PeerConnectionFactory::CreateVideoTrack can be used for creating a VideoTrack
+// that ensures thread safety and that all methods are called on the right
+// thread.
 class VideoTrackInterface
     : public MediaStreamTrackInterface,
       public rtc::VideoSourceInterface<VideoFrame> {
@@ -154,12 +165,6 @@
   // See https://crbug.com/653531 and https://github.com/WICG/mst-content-hint.
   enum class ContentHint { kNone, kFluid, kDetailed };
 
-  // Register a video sink for this track. Used to connect the track to the
-  // underlying video engine.
-  void AddOrUpdateSink(rtc::VideoSinkInterface<VideoFrame>* sink,
-                       const rtc::VideoSinkWants& wants) override {}
-  void RemoveSink(rtc::VideoSinkInterface<VideoFrame>* sink) override {}
-
   virtual VideoTrackSourceInterface* GetSource() const = 0;
 
   virtual ContentHint content_hint() const { return ContentHint::kNone; }