Replace layer id with Element id for tracking scrollbar animation controllers

cc/layers/layer.h

 // Copyright 2010 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_LAYERS_LAYER_H_
 #define CC_LAYERS_LAYER_H_
 
 #include <stddef.h>
 #include <stdint.h>
 
@@ skipping 385 matching lines @@
   }
 
   // This is for tracking damage.
   void SetSubtreePropertyChanged();
   bool subtree_property_changed() const { return subtree_property_changed_; }
 
   void SetMayContainVideo(bool yes);
 
   int num_copy_requests_in_target_subtree();
 
+  // Stable identifier for clients. See comment in cc/trees/element_id.h.
   void SetElementId(ElementId id);
   ElementId element_id() const { return inputs_.element_id; }
 
   void SetMutableProperties(uint32_t properties);
   uint32_t mutable_properties() const { return inputs_.mutable_properties; }
 
   bool HasTickingAnimationForTesting() const;
 
   void SetHasWillChangeTransformHint(bool has_will_change);
   bool has_will_change_transform_hint() const {
@@ skipping 221 matching lines @@
   // These all act like draw properties, so don't need push properties.
   gfx::Rect visible_layer_rect_;
   size_t num_unclipped_descendants_;
 
   DISALLOW_COPY_AND_ASSIGN(Layer);
 };
 
 }  // namespace cc
 
 #endif  // CC_LAYERS_LAYER_H_