Allow muting/unmuting audio through media track API

media/base/pipeline.h

Index: media/base/pipeline.h
diff --git a/media/base/pipeline.h b/media/base/pipeline.h
index db326c2928353f2aa9d6080ad650e4ecd56f22fb..07e197b98d772c0767e935c47dd5956eae8e4cf6 100644
--- a/media/base/pipeline.h
+++ b/media/base/pipeline.h
@@ -19,6 +19,7 @@
 namespace media {
 
 class Demuxer;
+class DemuxerStream;
 class Renderer;
 class VideoFrame;
 
@@ -155,6 +156,15 @@ class MEDIA_EXPORT Pipeline {
 
   virtual void SetCdm(CdmContext* cdm_context,
                       const CdmAttachedCB& cdm_attached_cb) = 0;
+
+  // Notifies pipeline that the set of enabled audio streams/tracks has changed.

[wolenetz, 2016/03/30 00:31:32]

Would these be better located in PipelineController? Or DemuxerHost?

Also, |enabledAudioStreams| can be an empty vector, which means audio is
disabled, right? Please add that to comment here, like you have for the video
method.

+  virtual void OnEnabledAudioStreamsChanged(
+      const std::vector<const DemuxerStream*>& enabledAudioStreams) {}
+
+  // Notifies pipeline that the selected video stream has changed. The input
+  // parameter |selectedVideoStream| can be null, which means video is disabled.
+  virtual void OnSelectedVideoStreamChanged(
+      const DemuxerStream* selectedVideoStream) {}
 };
 
 }  // namespace media