Allow muting/unmuting audio through media track API

media/base/media_tracks.h

Index: media/base/media_tracks.h
diff --git a/media/base/media_tracks.h b/media/base/media_tracks.h
index 4e0fbb0fc1f88195ce14cc05d14a424924be4d5d..6c3d5201b51ebc574d7bfd6272333b9d7e2dcc8d 100644
--- a/media/base/media_tracks.h
+++ b/media/base/media_tracks.h
@@ -27,17 +27,17 @@ class MEDIA_EXPORT MediaTracks {
   ~MediaTracks();
 
   // Callers need to ensure that track id is unique.
-  void AddAudioTrack(const AudioDecoderConfig& config,
-                     const std::string& id,
-                     const std::string& kind,
-                     const std::string& label,
-                     const std::string& language);
+  const MediaTrack* AddAudioTrack(const AudioDecoderConfig& config,
+                                  const std::string& id,
+                                  const std::string& kind,
+                                  const std::string& label,
+                                  const std::string& language);
   // Callers need to ensure that track id is unique.
-  void AddVideoTrack(const VideoDecoderConfig& config,
-                     const std::string& id,
-                     const std::string& kind,
-                     const std::string& label,
-                     const std::string& language);
+  const MediaTrack* AddVideoTrack(const VideoDecoderConfig& config,
+                                  const std::string& id,
+                                  const std::string& kind,
+                                  const std::string& label,
+                                  const std::string& language);
 
   const MediaTracksCollection& tracks() const { return tracks_; }