Replace layer id with Element id for tracking scrollbar animation controllers

cc/layers/painted_scrollbar_layer.h

Index: cc/layers/painted_scrollbar_layer.h
diff --git a/cc/layers/painted_scrollbar_layer.h b/cc/layers/painted_scrollbar_layer.h
index 8e1884f44f53671118101b62359ebcd26f49cfb6..5eacb1ec7e3e96073e67a787f8a4f1eaf0e01f4a 100644
--- a/cc/layers/painted_scrollbar_layer.h
+++ b/cc/layers/painted_scrollbar_layer.h
@@ -22,15 +22,17 @@ class CC_EXPORT PaintedScrollbarLayer : public ScrollbarLayerInterface,
 
   static scoped_refptr<PaintedScrollbarLayer> Create(
       std::unique_ptr<Scrollbar> scrollbar,
-      int scroll_layer_id);
+      int scroll_layer_id,
+      ElementId 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;
 
@@ -45,7 +47,8 @@ class CC_EXPORT PaintedScrollbarLayer : public ScrollbarLayerInterface,
 
  protected:
   PaintedScrollbarLayer(std::unique_ptr<Scrollbar> scrollbar,
-                        int scroll_layer_id);
+                        int scroll_layer_id,
+                        ElementId scroll_element_id);
   ~PaintedScrollbarLayer() override;
 
   // For unit tests
@@ -77,6 +80,7 @@ class CC_EXPORT PaintedScrollbarLayer : public ScrollbarLayerInterface,
 
   std::unique_ptr<Scrollbar> scrollbar_;
   int scroll_layer_id_;
+  ElementId scroll_element_id_;
 
   float internal_contents_scale_;
   gfx::Size internal_content_bounds_;