Initial implementation of AudioTrack, AudioTrackList, VideoTrack, and VideoTrackList.

public/platform/WebMediaPlayer.h

Index: public/platform/WebMediaPlayer.h
diff --git a/public/platform/WebMediaPlayer.h b/public/platform/WebMediaPlayer.h
index f329a926cbd2d1f578f8e096dd792256239171bb..7ea7800fffaead0ce37fc454781b41a2242a875a 100644
--- a/public/platform/WebMediaPlayer.h
+++ b/public/platform/WebMediaPlayer.h
@@ -162,6 +162,9 @@ public:
     virtual void exitFullscreen() { }
     // Returns true if the player can enter fullscreen.
     virtual bool canEnterFullscreen() const { return false; }
+
+    virtual void enabledAudioTracksChanged(const WebVector<WebString>& enabledTrackIDs) { }
+    virtual void selectedVideoTrackChanged(const WebString& selectedTrackID) { }
 };
 
 } // namespace blink