Refactor the TrayAudio code so that it can be used by other platforms.

ash/system/audio/audio_observer.h

+// Copyright 2014 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 ASH_SYSTEM_AUDIO_AUDIO_OBSERVER_H_
+#define ASH_SYSTEM_AUDIO_AUDIO_OBSERVER_H_
+
+namespace ash {
+
+class AudioObserver {
+ public:
+  virtual ~AudioObserver() {}
+
+  // Called when output volume changed.
+  virtual void OnOutputVolumeChanged() = 0;
+
+  // Called when output mute state changed.
+  virtual void OnOutputMuteChanged() = 0;
+
+  // Called when audio nodes changed.
+  virtual void OnAudioNodesChanged() = 0;
+
+  // Called when active audio output node changed.
+  virtual void OnActiveOutputNodeChanged() = 0;
+
+  // Called when active audio input node changed.
+  virtual void OnActiveInputNodeChanged() = 0;
+};
+
+}  // namespace ash
+
+#endif  // ASH_SYSTEM_AUDIO_AUDIO_OBSERVER_H_