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

 // Copyright (c) 2013 The Chromium Authors. All rights reserved.
 // Use of this source code is governed by a BSD-style license that can be
 // found in the LICENSE file.
 //
 // An AudioInputStream which provides a loop-back of all audio output generated
 // by the RenderView associated with a WebContents instance.  The single stream
 // of data is produced by format-converting and mixing all audio output from a
 // RenderView.  In other words, WebContentsAudioInputStream provides tab-level
 // audio mirroring.
 //
@@ skipping 37 matching lines @@
   virtual void SetVolume(double volume) OVERRIDE;
   virtual double GetVolume() OVERRIDE;
   virtual void SetAutomaticGainControl(bool enabled) OVERRIDE;
   virtual bool GetAutomaticGainControl() OVERRIDE;
 
   // Create a new audio mirroring session, or return NULL on error.  |device_id|
   // should be in the format accepted by
   // 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:
   friend class WebContentsAudioInputStreamTest;
 
   // Maintain most state and functionality in an internal ref-counted
   // implementation class.  This object must outlive a call to Close(), until
   // the shutdown tasks running on other threads complete: The
   // AudioMirroringManager on the IO thread, the WebContentsTracker on the UI
   // thread, and the VirtualAudioOuputStreams on the audio thread.
   class Impl;
 
   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);
 
   virtual ~WebContentsAudioInputStream();
 
   scoped_refptr<Impl> impl_;
 
   DISALLOW_COPY_AND_ASSIGN(WebContentsAudioInputStream);
 };
 
 }  // namespace content
 
 #endif  // CONTENT_BROWSER_RENDERER_HOST_MEDIA_WEB_CONTENTS_AUDIO_INPUT_STREAM_H_