Replace layer id with Element id for tracking scrollbar animation controllers

cc/trees/layer_tree_host_common.h

 // Copyright 2011 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_TREES_LAYER_TREE_HOST_COMMON_H_
 #define CC_TREES_LAYER_TREE_HOST_COMMON_H_
 
 #include <stddef.h>
 
 #include <limits>
@@ skipping 129 matching lines @@
 
     bool operator==(const ScrollUpdateInfo& other) const;
   };
 
   // Used to communicate scrollbar visibility from Impl thread to Blink.
   // Scrollbar input is handled by Blink but the compositor thread animates
   // opacity on scrollbars to fade them out when they're overlay. Blink needs
   // to be told when they're faded out so it can stop handling input for
   // invisible scrollbars.
   struct CC_EXPORT ScrollbarsUpdateInfo {
-    int layer_id;
+    ElementId element_id;
     bool hidden;
 
     ScrollbarsUpdateInfo();
-    ScrollbarsUpdateInfo(int layer_id, bool hidden);
+    ScrollbarsUpdateInfo(ElementId element_id, bool hidden);
 
     bool operator==(const ScrollbarsUpdateInfo& other) const;
   };
 };
 
 struct CC_EXPORT ScrollAndScaleSet {
   ScrollAndScaleSet();
   ~ScrollAndScaleSet();
 
   // The inner viewport scroll delta is kept separate since it's special.
@@ skipping 35 matching lines @@
     function(tree_impl->LayerById(id));
   }
 }
 
 CC_EXPORT PropertyTrees* GetPropertyTrees(Layer* layer);
 CC_EXPORT PropertyTrees* GetPropertyTrees(LayerImpl* layer);
 
 }  // namespace cc
 
 #endif  // CC_TREES_LAYER_TREE_HOST_COMMON_H_