Move ash tray audio UI code from ash/system/audio to ash/system/chromeos/audio.

ash/system/audio/audio_observer.h

-// Copyright (c) 2012 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_
-
-#include "ash/ash_export.h"
-
-namespace ash {
-
-class ASH_EXPORT AudioObserver {
- public:
-  virtual ~AudioObserver() {}
-
-  virtual void OnVolumeChanged(float percent) = 0;
-  virtual void OnMuteToggled() = 0;
-};
-
-}  // namespace ash
-
-#endif  //ASH_SYSTEM_AUDIO_AUDIO_OBSERVER_H_