Allow muting/unmuting audio through media track API

media/base/media_tracks.cc

Index: media/base/media_tracks.cc
diff --git a/media/base/media_tracks.cc b/media/base/media_tracks.cc
index 832e5a822b12a497f43d01aabc9ef898c55fb7d0..984ca1881baae653eeb6ca8c51f883413d8737e9 100644
--- a/media/base/media_tracks.cc
+++ b/media/base/media_tracks.cc
@@ -14,30 +14,32 @@ MediaTracks::MediaTracks() {}
 
 MediaTracks::~MediaTracks() {}
 
-void MediaTracks::AddAudioTrack(const AudioDecoderConfig& config,
-                                const std::string& id,
-                                const std::string& kind,
-                                const std::string& label,
-                                const std::string& language) {
+const MediaTrack* MediaTracks::AddAudioTrack(const AudioDecoderConfig& config,
+                                             const std::string& id,
+                                             const std::string& kind,
+                                             const std::string& label,
+                                             const std::string& language) {
   DCHECK(config.IsValidConfig());
   CHECK(audio_configs_.find(id) == audio_configs_.end());
   scoped_ptr<MediaTrack> track = make_scoped_ptr(
       new MediaTrack(MediaTrack::Audio, id, kind, label, language));
   tracks_.push_back(std::move(track));
   audio_configs_[id] = config;
+  return tracks_.back().get();
 }
 
-void MediaTracks::AddVideoTrack(const VideoDecoderConfig& config,
-                                const std::string& id,
-                                const std::string& kind,
-                                const std::string& label,
-                                const std::string& language) {
+const MediaTrack* MediaTracks::AddVideoTrack(const VideoDecoderConfig& config,
+                                             const std::string& id,
+                                             const std::string& kind,
+                                             const std::string& label,
+                                             const std::string& language) {
   DCHECK(config.IsValidConfig());
   CHECK(video_configs_.find(id) == video_configs_.end());
   scoped_ptr<MediaTrack> track = make_scoped_ptr(
       new MediaTrack(MediaTrack::Video, id, kind, label, language));
   tracks_.push_back(std::move(track));
   video_configs_[id] = config;
+  return tracks_.back().get();
 }
 
 const AudioDecoderConfig& MediaTracks::getAudioConfig(