Add mojo interface+impl for audio stream control.

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 used for one output stream.

[o1ka, 2017/03/07 23:31:07]

"for one output stream" is not quite clear, as if I can pass the same
|output_stream| into it multiple times.
"Can only be used once"?
and add "On error, the message pipe is closed."?

[Max Morin, 2017/03/08 07:38:29]

Done.
There aren't really any error that can occur (other than shutdown/removing
frame, in which case I think it's implicit that the message pipe is closed).

[o1ka, 2017/03/08 08:11:49]

Calling Acquire() twice?

+  Acquire(AudioOutputStream& output_stream, media.mojom.AudioParameters params) =>       (handle<shared_buffer> shared_buffer,        handle socket_descriptor);
+};