Add thread safety to AudioManagerBase to protect access to the audio thread member variable.

media/audio/audio_output_dispatcher.h

 // Copyright (c) 2012 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.
 
 // AudioOutputDispatcher is a single-threaded class that dispatches creation and
 // deletion of audio output streams. AudioOutputProxy objects use this class to
 // allocate and recycle actual audio output streams. When playback is started,
 // the proxy calls StreamStarted() to get an output stream that it uses to play
 // audio. When playback is stopped, the proxy returns the stream back to the
 // dispatcher by calling StreamStopped().
@@ skipping 51 matching lines @@
   // added to the pool of pending streams (i.e. |idle_streams_|).
   // Ownership of the |stream| is passed to the dispatcher.
   void StreamStopped(AudioOutputStream* stream);
 
   // Called by AudioOutputProxy when the stream is closed.
   void StreamClosed();
 
   // Called on the audio thread when the AudioManager is shutting down.
   void Shutdown();
 
-  MessageLoop* message_loop();
-
  private:
   friend class AudioOutputProxyTest;
 
   // Creates a new physical output stream, opens it and pushes to
   // |idle_streams_|.  Returns false if the stream couldn't be created or
   // opened.
   bool CreateAndOpenStream();
 
   // A task scheduled by StreamStarted(). Opens a new stream and puts
   // it in |idle_streams_|.
@@ skipping 19 matching lines @@
   AudioOutputStreamList pausing_streams_;
 
   // Used to post delayed tasks to ourselves that we cancel inside Shutdown().
   base::WeakPtrFactory<AudioOutputDispatcher> weak_this_;
   base::DelayTimer<AudioOutputDispatcher> close_timer_;
 
   DISALLOW_COPY_AND_ASSIGN(AudioOutputDispatcher);
 };
 
 #endif  // MEDIA_AUDIO_AUDIO_OUTPUT_DISPATCHER_H_