(Relanding) Restores larger output buffer size when output stream requiring smaller size is closed

media/audio/mac/audio_auhal_mac.h

 // Copyright 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.
 //
 // 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 AudioManagerMac::HardwareSampleRate() to retrieve the sample rate.
 // - Calling Close() also leads to self destruction.
@@ skipping 70 matching lines @@
   // Implementation of AudioOutputStream.
   bool Open() override;
   void Close() override;
   void Start(AudioSourceCallback* callback) override;
   void Stop() override;
   void SetVolume(double volume) override;
   void GetVolume(double* volume) override;
 
   AudioDeviceID device_id() const { return device_; }
   size_t requested_buffer_size() const { return number_of_frames_; }
+  AudioUnit audio_unit() const { return audio_unit_; }
 
  private:
   // AUHAL callback.
   static OSStatus InputProc(void* user_data,
                             AudioUnitRenderActionFlags* flags,
                             const AudioTimeStamp* time_stamp,
                             UInt32 bus_number,
                             UInt32 number_of_frames,
                             AudioBufferList* io_data);
 
@@ skipping 37 matching lines @@
   // Called from the dtor and when the stream is reset.
   void ReportAndResetStats();
 
   // Our creator, the audio manager needs to be notified when we close.
   AudioManagerMac* const manager_;
 
   const AudioParameters params_;
   // For convenience - same as in params_.
   const int output_channels_;
 
-  // Buffer-size.
+  // Size of audio buffer requested at construction. The actual buffer size
+  // is given by |actual_io_buffer_frame_size_| and it can differ from the
+  // requested size.
   const size_t number_of_frames_;
 
   // Stores the number of frames that we actually get callbacks for.
   // This may be different from what we ask for, so we use this for stats in
   // order to understand how often this happens and what are the typical values.
   size_t number_of_frames_requested_;
 
   // Pointer to the object that will provide the audio samples.
   AudioSourceCallback* source_;
 
@@ skipping 55 matching lines @@
   // Used to make sure control functions (Start(), Stop() etc) are called on the
   // right thread.
   base::ThreadChecker thread_checker_;
 
   DISALLOW_COPY_AND_ASSIGN(AUHALStream);
 };
 
 }  // namespace media
 
 #endif  // MEDIA_AUDIO_MAC_AUDIO_AUHAL_MAC_H_