Update from https://crrev.com/309717

cc/animation/layer_animation_event_observer.h

 // Copyright 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 CC_ANIMATION_LAYER_ANIMATION_EVENT_OBSERVER_H_
 #define CC_ANIMATION_LAYER_ANIMATION_EVENT_OBSERVER_H_
 
 #include "cc/base/cc_export.h"
 
 namespace cc {
 
+struct AnimationEvent;
+
 class CC_EXPORT LayerAnimationEventObserver {
  public:
   virtual void OnAnimationStarted(const AnimationEvent& event) = 0;
 
  protected:
   virtual ~LayerAnimationEventObserver() {}
 };
 
 }  // namespace cc
 
 #endif  // CC_ANIMATION_LAYER_ANIMATION_EVENT_OBSERVER_H_