ui: Cleanup class/struct forward declarations

ui/compositor/scoped_layer_animation_settings.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 UI_COMPOSITOR_SCOPED_LAYER_ANIMATION_SETTINGS_H_
 #define UI_COMPOSITOR_SCOPED_LAYER_ANIMATION_SETTINGS_H_
 
 #include <set>
 
 #include "base/macros.h"
 #include "base/time/time.h"
 #include "ui/compositor/compositor_export.h"
 #include "ui/compositor/layer_animator.h"
 #include "ui/gfx/animation/tween.h"
 
 namespace ui {
 
 class ImplicitAnimationObserver;
-class LayerAnimationObserver;
 
 // Scoped settings allow you to temporarily change the animator's settings and
 // these changes are reverted when the object is destroyed. NOTE: when the
 // settings object is created, it applies the default transition duration
 // (200ms).
 class COMPOSITOR_EXPORT ScopedLayerAnimationSettings {
  public:
   explicit ScopedLayerAnimationSettings(scoped_refptr<LayerAnimator> animator);
   virtual ~ScopedLayerAnimationSettings();
 
@@ skipping 21 matching lines @@
   gfx::Tween::Type old_tween_type_;
   LayerAnimator::PreemptionStrategy old_preemption_strategy_;
   std::set<ImplicitAnimationObserver*> observers_;
 
   DISALLOW_COPY_AND_ASSIGN(ScopedLayerAnimationSettings);
 };
 
 }  // namespace ui
 
 #endif  // UI_COMPOSITOR_SCOPED_LAYER_ANIMATION_SETTINGS_H_