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

webrtc/api/videotrack.h

Index: webrtc/api/videotrack.h
diff --git a/webrtc/api/videotrack.h b/webrtc/api/videotrack.h
index 57bc42fca79b3d22fb10d1af0e1a5088128d3bd8..02546efd6952645f87fde75a9b5bf2c452e38371 100644
--- a/webrtc/api/videotrack.h
+++ b/webrtc/api/videotrack.h
@@ -24,7 +24,8 @@
 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,
@@ -45,6 +46,9 @@ class VideoTrack : public MediaStreamTrack<VideoTrackInterface>,
   ~VideoTrack();
 
  private:
+  // Implements ObserverInterface. Observes |video_source_| state.
+  void OnChanged() override;
+
   rtc::ThreadChecker thread_checker_;
   rtc::scoped_refptr<VideoTrackSourceInterface> video_source_;
 };