Replace layer id with Element id for tracking scrollbar animation controllers

cc/layers/painted_overlay_scrollbar_layer.h

 // Copyright 2016 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_PAINTED_OVERLAY_SCROLLBAR_LAYER_H_
 #define CC_LAYERS_PAINTED_OVERLAY_SCROLLBAR_LAYER_H_
 
 #include "base/macros.h"
 #include "cc/cc_export.h"
 #include "cc/input/scrollbar.h"
 #include "cc/layers/layer.h"
 #include "cc/layers/scrollbar_layer_interface.h"
 #include "cc/layers/scrollbar_theme_painter.h"
 #include "cc/resources/scoped_ui_resource.h"
 
 namespace cc {
 
 class CC_EXPORT PaintedOverlayScrollbarLayer : public ScrollbarLayerInterface,
                                                public Layer {
  public:
   std::unique_ptr<LayerImpl> CreateLayerImpl(LayerTreeImpl* tree_impl) override;
 
   static scoped_refptr<PaintedOverlayScrollbarLayer> Create(
       std::unique_ptr<Scrollbar> scrollbar,
-      int scroll_layer_id);
+      int scroll_layer_id,
+      ElementId scroll_element_id = ElementId());
 
   bool OpacityCanAnimateOnImplThread() const override;
   bool AlwaysUseActiveTreeOpacity() const override;
   ScrollbarLayerInterface* ToScrollbarLayer() override;
 
   // ScrollbarLayerInterface
-  int ScrollLayerId() const override;
-  void SetScrollLayer(int layer_id) override;
+  ElementId scroll_element_id() const override;
+  // TODO(pdr): Remove layer_id and refactor scrollbars to just use element ids.
+  void SetScrollInfo(int layer_id, ElementId element_id) override;
   ScrollbarOrientation orientation() const override;
 
   // Layer interface
   bool Update() override;
   void SetLayerTreeHost(LayerTreeHost* host) override;
   void PushPropertiesTo(LayerImpl* layer) override;
 
  protected:
   PaintedOverlayScrollbarLayer(std::unique_ptr<Scrollbar> scrollbar,
-                               int scroll_layer_id);
+                               int scroll_layer_id,
+                               ElementId scroll_element_id);
   ~PaintedOverlayScrollbarLayer() override;
 
  private:
   gfx::Rect OriginThumbRectForPainting() const;
 
   template <typename T>
   bool UpdateProperty(T value, T* prop) {
     if (*prop == value)
       return false;
     *prop = value;
     SetNeedsPushProperties();
     return true;
   }
 
   bool PaintThumbIfNeeded();
 
   std::unique_ptr<Scrollbar> scrollbar_;
   int scroll_layer_id_;
+  ElementId scroll_element_id_;
 
   int thumb_thickness_;
   int thumb_length_;
   gfx::Point location_;
   gfx::Rect track_rect_;
 
   gfx::Rect aperture_;
 
   std::unique_ptr<ScopedUIResource> thumb_resource_;
 
   DISALLOW_COPY_AND_ASSIGN(PaintedOverlayScrollbarLayer);
 };
 
 }  // namespace cc
 
 #endif  // CC_LAYERS_PAINTED_OVERLAY_SCROLLBAR_LAYER_H_