chromeos: Cleanup class/struct fwd declarations

chromeos/audio/audio_pref_observer.h

 // Copyright (c) 2013 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 CHROMEOS_AUDIO_AUDIO_PREF_OBSERVER_H_
 #define CHROMEOS_AUDIO_AUDIO_PREF_OBSERVER_H_
 
 #include "base/memory/ref_counted.h"
 #include "chromeos/chromeos_export.h"
 
-class PrefRegistrySimple;
-
 namespace chromeos {
 
 // Interface for observing audio preference changes.
 class CHROMEOS_EXPORT AudioPrefObserver {
  public:
   // Called when audio policy prefs changed.
   virtual void OnAudioPolicyPrefChanged() = 0;
 
  protected:
   virtual ~AudioPrefObserver() {}
 };
 
 }  // namespace chromeos
 
 #endif  // CHROMEOS_AUDIO_AUDIO_PREF_OBSERVER_H_