Propagate MediaStreamSource state to video tracks the same way as audio.

webrtc/api/videotrack.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.
  */
 
 #ifndef WEBRTC_API_VIDEOTRACK_H_
 #define WEBRTC_API_VIDEOTRACK_H_
 
 #include <string>
 #include <vector>
 
 #include "webrtc/api/mediastreamtrack.h"
 #include "webrtc/base/scoped_ref_ptr.h"
 #include "webrtc/base/thread_checker.h"
 #include "webrtc/media/base/videosourcebase.h"
 
 namespace webrtc {
 
 class VideoTrack : public MediaStreamTrack<VideoTrackInterface>,
-                   public rtc::VideoSourceBase {
+                   public rtc::VideoSourceBase,
+                   public ObserverInterface {
  public:
   static rtc::scoped_refptr<VideoTrack> Create(
       const std::string& label,
       VideoTrackSourceInterface* source);
 
   void AddOrUpdateSink(rtc::VideoSinkInterface<cricket::VideoFrame>* sink,
                        const rtc::VideoSinkWants& wants) override;
   void RemoveSink(rtc::VideoSinkInterface<cricket::VideoFrame>* sink) override;
 
   virtual VideoTrackSourceInterface* GetSource() const {
     return video_source_.get();
   }
   virtual bool set_enabled(bool enable);
   virtual std::string kind() const;
 
  protected:
   VideoTrack(const std::string& id, VideoTrackSourceInterface* video_source);
   ~VideoTrack();
 
  private:
+  // Implements ObserverInterface. Observes |video_source_| state.
+  void OnChanged() override;
+
   rtc::ThreadChecker thread_checker_;
   rtc::scoped_refptr<VideoTrackSourceInterface> video_source_;
 };
 
 }  // namespace webrtc
 
 #endif  // WEBRTC_API_VIDEOTRACK_H_