cc: LayerTreeHostImpl uses element id to tick animations

cc/layers/scrollbar_layer_impl_base.h

 // Copyright 2013 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_SCROLLBAR_LAYER_IMPL_BASE_H_
 #define CC_LAYERS_SCROLLBAR_LAYER_IMPL_BASE_H_
 
 #include "base/macros.h"
 #include "cc/cc_export.h"
 #include "cc/input/scrollbar.h"
@@ skipping 38 matching lines @@
   // Thumb quad rect in layer space.
   virtual gfx::Rect ComputeThumbQuadRect() const;
 
   float thumb_thickness_scale_factor() {
     return thumb_thickness_scale_factor_;
   }
   bool SetThumbThicknessScaleFactor(float thumb_thickness_scale_factor);
 
   virtual int ThumbThickness() const = 0;
 
+  // TODO(crbug.com/702832): No need for this function once there is element id
+  // on overlay scrollbar layers.
+  void SetOverlayScrollbarLayerOpacityAnimated(float opacity);
+
  protected:
   ScrollbarLayerImplBase(LayerTreeImpl* tree_impl,
                          int id,
                          ScrollbarOrientation orientation,
                          bool is_left_side_vertical_scrollbar,
                          bool is_overlay);
   ~ScrollbarLayerImplBase() override;
 
   virtual int ThumbLength() const = 0;
   virtual float TrackLength() const = 0;
@@ skipping 18 matching lines @@
   float vertical_adjust_;
 
   DISALLOW_COPY_AND_ASSIGN(ScrollbarLayerImplBase);
 };
 
 typedef std::set<ScrollbarLayerImplBase*> ScrollbarSet;
 
 }  // namespace cc
 
 #endif  // CC_LAYERS_SCROLLBAR_LAYER_IMPL_BASE_H_