Replace layer id with Element id for tracking scrollbar animation controllers

cc/layers/layer_impl.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_LAYERS_LAYER_IMPL_H_
 #define CC_LAYERS_LAYER_IMPL_H_
 
 #include <stddef.h>
 #include <stdint.h>
 
@@ skipping 164 matching lines @@
 
   void SetMasksToBounds(bool masks_to_bounds);
   bool masks_to_bounds() const { return masks_to_bounds_; }
 
   void SetContentsOpaque(bool opaque);
   bool contents_opaque() const { return contents_opaque_; }
 
   float Opacity() const;
   const gfx::Transform& Transform() const;
 
+  // Stable identifier for clients. See comment in cc/trees/element_id.h.
   void SetElementId(ElementId element_id);
   ElementId element_id() const { return element_id_; }
 
   void SetMutableProperties(uint32_t properties);
   uint32_t mutable_properties() const { return mutable_properties_; }
 
   void SetPosition(const gfx::PointF& position);
   gfx::PointF position() const { return position_; }
 
   bool IsAffectedByPageScale() const;
@@ skipping 361 matching lines @@
   // the overlay scrollbars. It's set on the scroll layer (not the scrollbar
   // layers) and consumed by LayerTreeImpl::PushPropertiesTo during activation.
   bool needs_show_scrollbars_ : 1;
 
   DISALLOW_COPY_AND_ASSIGN(LayerImpl);
 };
 
 }  // namespace cc
 
 #endif  // CC_LAYERS_LAYER_IMPL_H_