Replace layer id with Element id for tracking scrollbar animation controllers

cc/layers/painted_overlay_scrollbar_layer.h

Index: cc/layers/painted_overlay_scrollbar_layer.h
diff --git a/cc/layers/painted_overlay_scrollbar_layer.h b/cc/layers/painted_overlay_scrollbar_layer.h
index f7cd66e5fd52a0ada74f33939581e815d45c6f1d..c71f7b5988a20acd3533587c5420d3f969ab792d 100644
--- a/cc/layers/painted_overlay_scrollbar_layer.h
+++ b/cc/layers/painted_overlay_scrollbar_layer.h
@@ -22,15 +22,17 @@ class CC_EXPORT PaintedOverlayScrollbarLayer : public ScrollbarLayerInterface,
 
   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
@@ -40,7 +42,8 @@ class CC_EXPORT PaintedOverlayScrollbarLayer : public ScrollbarLayerInterface,
 
  protected:
   PaintedOverlayScrollbarLayer(std::unique_ptr<Scrollbar> scrollbar,
-                               int scroll_layer_id);
+                               int scroll_layer_id,
+                               ElementId scroll_element_id);
   ~PaintedOverlayScrollbarLayer() override;
 
  private:
@@ -59,6 +62,7 @@ class CC_EXPORT PaintedOverlayScrollbarLayer : public ScrollbarLayerInterface,
 
   std::unique_ptr<Scrollbar> scrollbar_;
   int scroll_layer_id_;
+  ElementId scroll_element_id_;
 
   int thumb_thickness_;
   int thumb_length_;