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

webrtc/api/mediastreaminterface.h

Index: webrtc/api/mediastreaminterface.h
diff --git a/webrtc/api/mediastreaminterface.h b/webrtc/api/mediastreaminterface.h
index cc660e1be239e285a8f4124b417d458e283673f5..aaf5b62aa42ac38eb4cb5843215ab8ee96a15cc3 100644
--- a/webrtc/api/mediastreaminterface.h
+++ b/webrtc/api/mediastreaminterface.h
@@ -111,6 +111,11 @@ class MediaStreamTrackInterface : public rtc::RefCountInterface,
 
 // 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 @@ class VideoTrackSourceInterface
   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> {