Add MuteableAudioOutputStream interface to export SetMute().

media/audio/android/muteable_audio_output_stream.h

+// 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.
+
+#ifndef MEDIA_MUTEABLE_AUDIO_OUTPUT_STREAM_H_
+#define MEDIA_MUTEABLE_AUDIO_OUTPUT_STREAM_H_
+
+#include "media/audio/audio_io.h"
+
+namespace media {
+
+class MEDIA_EXPORT MuteableAudioOutputStream : public AudioOutputStream {
+ public:
+  // Volume control coming from hardware. It overrides volume when it's
+  // true. Otherwise, use SetVolume(double volume) for scaling.
+  // This is needed because platform voice volume never goes to zero in
+  // COMMUNICATION mode on Android.
+  virtual void SetMute(bool muted) = 0;
+};
+
+}  // namespace media
+
+#endif  // MEDIA_MUTEABLE_AUDIO_OUTPUT_STREAM_H_