Change audio render thread checking to use new AudioRendererSink::BelongsToRendererThread()

media/base/audio_renderer_sink.h

Index: media/base/audio_renderer_sink.h
diff --git a/media/base/audio_renderer_sink.h b/media/base/audio_renderer_sink.h
index b0b5f26a729d3667882ded2265250250430b7c9e..4e32f741be7837965c63212d3362d3e4bb6a1770 100644
--- a/media/base/audio_renderer_sink.h
+++ b/media/base/audio_renderer_sink.h
@@ -27,6 +27,10 @@ class AudioRendererSink
  public:
   class RenderCallback {
    public:
+    // Informs that rendering has been initialized and will start soon. This
+    // means that a new rendering thread has been started.
+    virtual void OnInitialized() = 0;

[Henrik Grunell, 2016/06/03 13:53:42]

Only WebRTC will care about this function, and will make sure
MediaStreamAudioProcessor detaches from the thread it's currently bound to for
render calls.

[DaleCurtis, 2016/06/03 20:54:13]

Why not always detach until the first render call?

[Henrik Grunell, 2016/06/08 09:19:06]

The mixer in the upcoming mixing changes will restart the AOD (thus starting a
new thread) if needed when a new input is added. We need to be informed when
that happens and this is the means to do that. OnInitialized() is perhaps not an
accurate naming, but I'd like something somewhat generic.

+
     // Attempts to completely fill all channels of |dest|, returns actual
     // number of frames filled. |frames_skipped| contains the number of frames
     // the consumer has skipped, if any.
@@ -70,6 +74,9 @@ class AudioRendererSink
   // Must never be called on the IO thread.
   virtual OutputDeviceInfo GetOutputDeviceInfo() = 0;
 
+  // Returns true if called on rendering thread, otherwise false.
+  virtual bool BelongsToRenderingThread() = 0;

[Henrik Grunell, 2016/06/03 13:53:42]

If we go for the above OnInitialized(), we don't need this function.

If we keep this function instead, only WebRTC will use it. In that case we can't
detach in MSAP. We have two options: remove thread checks in MSAP or forward
thread checking to elsewhere (i.e. its owner).

[DaleCurtis, 2016/06/03 20:54:13]

What do you mean about detaching? This would replace any threading checks, no?

[Henrik Grunell, 2016/06/08 09:19:06]

In the case above, when the mixer restarts AOD, with this option we won't know
about that and not be able to detach. Yes, this would replace thread checks.

[DaleCurtis, 2016/06/08 18:10:13]

The corollary to my question is that if this replaces thread checks, why do you
even need to detach? Just always use this instead of a ThreadChecker.

+
  protected:
   friend class base::RefCountedThreadSafe<AudioRendererSink>;
   virtual ~AudioRendererSink() {}