Make rendering of MediaStreams reflect changes to its set of tracks.

content/renderer/media/webmediaplayer_ms.h

Index: content/renderer/media/webmediaplayer_ms.h
diff --git a/content/renderer/media/webmediaplayer_ms.h b/content/renderer/media/webmediaplayer_ms.h
index 8fc8a04e7dbe3057a64d3448c379f19055c8d785..6c50a409fbbf551b68cbcc69a3f56f3bc0bf6675 100644
--- a/content/renderer/media/webmediaplayer_ms.h
+++ b/content/renderer/media/webmediaplayer_ms.h
@@ -14,6 +14,7 @@
 #include "base/synchronization/lock.h"
 #include "base/threading/thread_checker.h"
 #include "content/common/content_export.h"
+#include "content/renderer/media/media_stream.h"
 #include "media/blink/webmediaplayer_delegate.h"
 #include "media/blink/webmediaplayer_util.h"
 #include "media/renderers/gpu_video_accelerator_factories.h"
@@ -62,7 +63,8 @@ class WebMediaPlayerMSCompositor;
 // blink::WebMediaPlayerClient
 //   WebKit client of this media player object.
 class CONTENT_EXPORT WebMediaPlayerMS
-    : public NON_EXPORTED_BASE(blink::WebMediaPlayer),
+    : public NON_EXPORTED_BASE(MediaStreamObserver),
+      public NON_EXPORTED_BASE(blink::WebMediaPlayer),
       public NON_EXPORTED_BASE(media::WebMediaPlayerDelegate::Observer),
       public NON_EXPORTED_BASE(base::SupportsWeakPtr<WebMediaPlayerMS>) {
  public:
@@ -177,6 +179,10 @@ class CONTENT_EXPORT WebMediaPlayerMS
                     bool flip_y,
                     bool premultiply_alpha) override;
 
+  // MediaStreamObserver implementation
+  void TrackAdded(const blink::WebMediaStreamTrack& track) override;
+  void TrackRemoved(const blink::WebMediaStreamTrack& track) override;
+
  private:
   friend class WebMediaPlayerMSTest;
 
@@ -199,6 +205,9 @@ class CONTENT_EXPORT WebMediaPlayerMS
   // Getter method to |client_|.
   blink::WebMediaPlayerClient* get_client() { return client_; }
 
+  // To be run when tracks are added or removed.
+  void Reload();
+
   blink::WebLocalFrame* const frame_;
 
   blink::WebMediaPlayer::NetworkState network_state_;
@@ -264,6 +273,7 @@ class CONTENT_EXPORT WebMediaPlayerMS
   // True if playback should be started upon the next call to OnShown(). Only
   // used on Android.
   bool should_play_upon_shown_;
+  blink::WebMediaStream web_stream_;
 
   DISALLOW_COPY_AND_ASSIGN(WebMediaPlayerMS);
 };