Move audio messages to their own file.

content/common/audio_stream_state.h

+// Copyright (c) 2011 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 CONTENT_COMMON_AUDIO_STREAM_STATE_H_
+#define CONTENT_COMMON_AUDIO_STREAM_STATE_H_
+#pragma once
+
+// Current status of the audio output stream in the browser process. Browser
+// sends information about the current playback state and error to the
+// renderer process using this type.
+enum AudioStreamState {
+  kAudioStreamPlaying,
+  kAudioStreamPaused,
+  kAudioStreamError
+};
+
+#endif  // CONTENT_COMMON_AUDIO_STREAM_STATE_H_