fix crash in AUAudioOutputStream::Render()

media/audio/mac/audio_low_latency_output_mac.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.
 //
 // Implementation notes:
 //
 // - It is recommended to first acquire the native sample rate of the default
 //   output device and then use the same rate when creating this object.
 //   Use AUAudioOutputStream::HardwareSampleRate() to retrieve the sample rate.
 // - Calling Close() also leads to self destruction.
@@ skipping 75 matching lines @@
 
   // The UID refers to the current output audio device.
   AudioDeviceID output_device_id_;
 
   // Volume level from 0 to 1.
   float volume_;
 
   // Fixed playout hardware latency in frames.
   double hardware_latency_frames_;
 
+  // The flag used to stop the streaming.
+  bool stopped_;
+
   DISALLOW_COPY_AND_ASSIGN(AUAudioOutputStream);
 };
 
 #endif  // MEDIA_AUDIO_MAC_AUDIO_LOW_LATENCY_OUTPUT_MAC_H_