start/stop the source of the capturer when 1st audiotrack/last audiotrack is added/removed

content/renderer/media/webrtc_local_audio_track.h

Index: content/renderer/media/webrtc_local_audio_track.h
diff --git a/content/renderer/media/webrtc_local_audio_track.h b/content/renderer/media/webrtc_local_audio_track.h
index 660d6969f1874b9eeb9589705d6115c93aa1269b..7a4eeb72fdda59f9d2f79d33a642d02bf63143c2 100644
--- a/content/renderer/media/webrtc_local_audio_track.h
+++ b/content/renderer/media/webrtc_local_audio_track.h
@@ -44,6 +44,14 @@ class CONTENT_EXPORT WebRtcLocalAudioTrack
   // Called on the main render thread.
   void RemoveSink(WebRtcAudioCapturerSink* sink);
 
+  // Starts the local audio track. Called on the main render thread and
+  // should be called only once when audio track is created.
+  void Start();
+
+  // Stops the local audio track. Called on the main render thread and
+  // should be called only once when audio track going away.
+  void Stop();
+
  protected:
   WebRtcLocalAudioTrack(const std::string& label,
                         const scoped_refptr<WebRtcAudioCapturer>& capturer,