Add a mojo implementation of AudioOutputIPC.

media/mojo/interfaces/audio_output_stream.mojom

 // Copyright 2016 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.
 
 module media.mojom;
 
 import "media/mojo/interfaces/audio_parameters.mojom";
 import "media/mojo/interfaces/media_types.mojom";
 
 // An interface for controlling an audio output stream.
 // On error, the message pipe is closed.
 // To close the stream, just close the message pipe.
 interface AudioOutputStream {
   // Starts rendering audio.
   Play();
 
   // Stops rendering audio and sends a signal to the |socket_descriptor|
   // indicating this.
   Pause();
 
   // Sets volume. Volume must be in the range [0, 1].
   SetVolume(double volume);
 };
 
 interface AudioOutputStreamProvider {
   // Creates a new AudioOutputStream using |params|. |shared_buffer| and
   // |socket_descriptor| are used to write data to the stream as defined
-  // in AudioDeviceThread. Can only be called once.
+  // in AudioDeviceThread. This means:
+  // * |shared_buffer| will be a writable handle to memory of the size needed
+  //   by AudioDeviceThread for the specified params.
+  // * |socket_descriptor| is a descriptor from which a
+  //   base::CancelableSyncSocket can be constructed.
+  // Can only be called once.
   Acquire(AudioOutputStream& output_stream, AudioParameters params)
   => (handle<shared_buffer> shared_buffer, handle socket_descriptor);
 };