Use element id's for animations

cc/input/scroll_state_data.h

 // Copyright 2015 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_INPUT_SCROLL_STATE_DATA_H_
 #define CC_INPUT_SCROLL_STATE_DATA_H_
 
 #include <stdint.h>
 
 #include <list>
@@ skipping 39 matching lines @@
   // consuming delta has caused a scroll. Currently, there are some cases where
   // we consume delta without scrolling, such as in
   // |Viewport::AdjustOverscroll|. Once these cases are fixed, we should get rid
   // of |caused_scroll_*_|. See crbug.com/510045 for details.
   bool caused_scroll_x;
   bool caused_scroll_y;
 
   ScrollNode* current_native_scrolling_node() const;
   void set_current_native_scrolling_node(
       ScrollNode* current_native_scrolling_node);
-  uint64_t current_native_scrolling_element() const;
-  void set_current_native_scrolling_element(uint64_t element_id);
+  ElementId current_native_scrolling_element() const;
+  void set_current_native_scrolling_element(ElementId element_id);
 
  private:
   // Only one of current_native_scrolling_node_ and
   // current_native_scrolling_element_ may be non-null at a time. Whenever
   // possible, we should store the scroll node.
 
   // The last scroll node to respond to a scroll, or null if none exists.
   ScrollNode* current_native_scrolling_node_;
   // The id of the last native element to respond to a scroll, or 0 if none
   // exists.
-  uint64_t current_native_scrolling_element_;
+  ElementId current_native_scrolling_element_;
 };
 
 }  // namespace cc
 
 #endif  // CC_INPUT_SCROLL_STATE_DATA_H_