Re-land r207105 (Mac audio capture threading fix) with unit test memory leak issue resolved.

content/browser/renderer_host/media/web_contents_audio_input_stream.h

Index: content/browser/renderer_host/media/web_contents_audio_input_stream.h
diff --git a/content/browser/renderer_host/media/web_contents_audio_input_stream.h b/content/browser/renderer_host/media/web_contents_audio_input_stream.h
index 2e8fd32a07d3b4d915cbb14e898321aacd6a5bbe..486547ecaa217b9ad8460adfd818a40972e9f03e 100644
--- a/content/browser/renderer_host/media/web_contents_audio_input_stream.h
+++ b/content/browser/renderer_host/media/web_contents_audio_input_stream.h
@@ -55,10 +55,13 @@ class CONTENT_EXPORT WebContentsAudioInputStream
   // WebContentsCaptureUtil::ExtractTabCaptureTarget().  The caller must
   // guarantee Close() is called on the returned object so that it may
   // self-destruct.
+  // |worker_loop| is the loop on which AudioInputCallback methods are called
+  // and may or may not be the single thread that invokes the AudioInputStream
+  // methods.
   static WebContentsAudioInputStream* Create(
       const std::string& device_id,
       const media::AudioParameters& params,
-      const scoped_refptr<base::MessageLoopProxy>& message_loop,
+      const scoped_refptr<base::MessageLoopProxy>& worker_loop,
       AudioMirroringManager* audio_mirroring_manager);
 
  private:
@@ -73,7 +76,6 @@ class CONTENT_EXPORT WebContentsAudioInputStream
 
   WebContentsAudioInputStream(
       int render_process_id, int render_view_id,
-      const scoped_refptr<base::MessageLoopProxy>& message_loop,
       AudioMirroringManager* mirroring_manager,
       const scoped_refptr<WebContentsTracker>& tracker,
       media::VirtualAudioInputStream* mixer_stream);