Use element id's for animations

cc/animation/scroll_offset_animations.h

 // Copyright (c) 2016 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_SCROLL_OFFSET_ANIMATIONS_H_
 #define CC_ANIMATION_SCROLL_OFFSET_ANIMATIONS_H_
 
 #include <unordered_map>
 
 #include "cc/animation/scroll_offset_animations_impl.h"
@@ skipping 33 matching lines @@
   bool HasUpdatesForTesting() const;
 
   // Goes through the updates in the order in which they're added and calls the
   // appropriate handler for each update.
   void PushPropertiesTo(ScrollOffsetAnimationsImpl*);
 
  private:
   ScrollOffsetAnimationUpdate GetUpdateForElementId(ElementId) const;
 
   using ElementToUpdateMap =
-      std::unordered_map<ElementId, ScrollOffsetAnimationUpdate>;
+      std::unordered_map<ElementId, ScrollOffsetAnimationUpdate, ElementIdHash>;
   ElementToUpdateMap element_to_update_map_;
 
   AnimationHost* animation_host_;
 };
 
 }  // namespace cc
 
 #endif  // CC_ANIMATION_SCROLL_OFFSET_ANIMATIONS_H_