MediaStream audio object graph untangling and clean-ups.

content/renderer/media/webrtc/webrtc_local_audio_track_adapter.h

Index: content/renderer/media/webrtc/webrtc_local_audio_track_adapter.h
diff --git a/content/renderer/media/webrtc/webrtc_local_audio_track_adapter.h b/content/renderer/media/webrtc/webrtc_local_audio_track_adapter.h
index cfe4a98ef9d291d96e0746833ecd3edcf2dab27e..36f5bda0cf9d5ddac8aed8295711b7541becc5a4 100644
--- a/content/renderer/media/webrtc/webrtc_local_audio_track_adapter.h
+++ b/content/renderer/media/webrtc/webrtc_local_audio_track_adapter.h
@@ -13,6 +13,7 @@
 #include "base/synchronization/lock.h"
 #include "base/threading/thread_checker.h"
 #include "content/common/content_export.h"
+#include "content/renderer/media/media_stream_audio_level_calculator.h"
 #include "third_party/webrtc/api/mediastreamtrack.h"
 #include "third_party/webrtc/media/base/audiorenderer.h"
 
@@ -31,6 +32,10 @@ class MediaStreamAudioProcessor;
 class WebRtcAudioSinkAdapter;
 class WebRtcLocalAudioTrack;
 
+// Provides an implementation of the webrtc::AudioTrackInterface that can be
+// bound/unbound to/from a MediaStreamAudioTrack.  In other words, this is an
+// adapter that sits between the media stream object graph and WebRtc's object
+// graph and proxies between the two.
 class CONTENT_EXPORT WebRtcLocalAudioTrackAdapter
     : NON_EXPORTED_BASE(
           public webrtc::MediaStreamTrack<webrtc::AudioTrackInterface>) {
@@ -48,9 +53,9 @@ class CONTENT_EXPORT WebRtcLocalAudioTrackAdapter
 
   void Initialize(WebRtcLocalAudioTrack* owner);
 
-  // Called on the audio thread by the WebRtcLocalAudioTrack to set the signal
-  // level of the audio data.
-  void SetSignalLevel(int signal_level);
+  // Set the object that provides shared access to the current audio signal
+  // level.
+  void SetLevel(scoped_refptr<MediaStreamAudioLevelCalculator::Level> level);
 
   // Method called by the WebRtcLocalAudioTrack to set the processor that
   // applies signal processing on the data of the track.
@@ -93,8 +98,8 @@ class CONTENT_EXPORT WebRtcLocalAudioTrackAdapter
   // from the audio track.
   ScopedVector<WebRtcAudioSinkAdapter> sink_adapters_;
 
-  // The amplitude of the signal.
-  int signal_level_;
+  // Thread-safe accessor to current audio signal level.

[mcasas, 2016/02/26 01:28:19]

If this is thread-safe, we don't need to protect it with
the |lock_| in l.109 right? Rename it if so to e.g.
|thread_safe_level_calculator_|.

[miu, 2016/02/27 03:46:36]

Done.  Actually, that's an interesting point.  If this refptr starts out as
null, and is only assigned once, it is completely safe to mutate it and read it
without a lock.

I also noticed, with a little more clean-up in WebRtcAudioCapturer, I was able
to do the same for |audio_processor_|.  So, now we don't need a lock for that.

...and now |lock_| isn't needed at all anymore.  So, I removed that.

+  scoped_refptr<MediaStreamAudioLevelCalculator::Level> level_;
 
   // Thread checker for libjingle's signaling thread.
   base::ThreadChecker signaling_thread_checker_;

[mcasas, 2016/02/26 01:28:19]

We don't need |signaling_thread_checker_| is we have
|signaling_thread_| on which we can call 
BelongsToCurrentThread() and  we assume is not nullptr 
on ctor (needs at least a DCHECK btw).

[miu, 2016/02/27 03:46:36]

Done.  Removed both since the latter is only used in one place (and is detached
there!).