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

ash/system/audio/audio_observer.h

Index: ash/system/audio/audio_observer.h
diff --git a/ash/system/audio/audio_observer.h b/ash/system/audio/audio_observer.h
deleted file mode 100644
index d0ff405c42076c0a2b7bbb8ab37a21c2a2aecbd3..0000000000000000000000000000000000000000
--- a/ash/system/audio/audio_observer.h
+++ /dev/null
@@ -1,22 +0,0 @@
-// 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_