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..f9862f5d82304612ea399c042e4f97fa7e330e48 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);
 
+  // Start the local audio track. Called on the main render thread and

[henrika (OOO until Aug 14), 2013/06/05 09:09:20]

nit, Starts

[no longer working on chromium, 2013/06/05 16:29:45]

Done.

+  // should be called only once when audio track is created.
+  void Start();
+
+  // Stop the local audio track. Called on the main render thread and
+  // should be called only once when audio track going away.

[henrika (OOO until Aug 14), 2013/06/05 09:09:20]

Who is responsible for ensuring that it is only called once? What happens if it
is called more?

[no longer working on chromium, 2013/06/05 16:29:45]

Good question, in the real life, Stop() will be called when JS calls stop() on
the audio track, and I assume the audio track will be invalid after that, so
they can't be called once.

Anyhow, I agree that it should be allowed to be called multiple times, and only
the first one has effect.

+  void Stop();
+
  protected:
   WebRtcLocalAudioTrack(const std::string& label,
                         const scoped_refptr<WebRtcAudioCapturer>& capturer,