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

webrtc/api/mediastreaminterface.h

 /*
  *  Copyright 2012 The WebRTC project authors. All Rights Reserved.
  *
  *  Use of this source code is governed by a BSD-style license
  *  that can be found in the LICENSE file in the root of the source
  *  tree. An additional intellectual property rights grant can be found
  *  in the file PATENTS.  All contributing project authors may
  *  be found in the AUTHORS file in the root of the source tree.
  */
 
@@ skipping 93 matching lines @@
 
   // Live or ended. A track will never be live again after becoming ended.
   virtual TrackState state() const = 0;
 
  protected:
   virtual ~MediaStreamTrackInterface() {}
 };
 
 // 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> {
  public:
   struct Stats {
     // Original size of captured frame, before video adaptation.
     int input_width;
     int input_height;
   };
 
@@ skipping 14 matching lines @@
   // Returns false if no stats are available, e.g, for a remote source, or a
   // source which has not seen its first frame yet.
   //
   // Implementation should avoid blocking.
   virtual bool GetStats(Stats* stats) = 0;
 
  protected:
   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> {
  public:
   // Video track content hint, used to override the source is_screencast
   // property.
   // 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
@@ skipping 145 matching lines @@
   virtual bool RemoveTrack(AudioTrackInterface* track) = 0;
   virtual bool RemoveTrack(VideoTrackInterface* track) = 0;
 
  protected:
   virtual ~MediaStreamInterface() {}
 };
 
 }  // namespace webrtc
 
 #endif  // WEBRTC_API_MEDIASTREAMINTERFACE_H_