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 36b03b80e43d62d25ef005432d890b9dae052cd2..3835d2c936eddd0f305f1867fd60e6fcbc23eca9 100644
--- a/webrtc/api/videotrack.h
+++ b/webrtc/api/videotrack.h
@@ -22,7 +22,8 @@
 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,
@@ -43,6 +44,9 @@ 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_;
 };