Replace layer id with Element id for tracking scrollbar animation controllers

cc/test/fake_painted_scrollbar_layer.h

Index: cc/test/fake_painted_scrollbar_layer.h
diff --git a/cc/test/fake_painted_scrollbar_layer.h b/cc/test/fake_painted_scrollbar_layer.h
index bbb38cdfb8fad9c9260b61c0f990e0195f92fedc..fe760b26cd6c0d9a21115875647b9498bb4177f3 100644
--- a/cc/test/fake_painted_scrollbar_layer.h
+++ b/cc/test/fake_painted_scrollbar_layer.h
@@ -21,14 +21,16 @@ class FakePaintedScrollbarLayer : public PaintedScrollbarLayer {
   static scoped_refptr<FakePaintedScrollbarLayer> Create(
       bool paint_during_update,
       bool has_thumb,
-      int scrolling_layer_id);
+      int scrolling_layer_id,
+      ElementId scrolling_element_id = ElementId());
   static scoped_refptr<FakePaintedScrollbarLayer> Create(
       bool paint_during_update,
       bool has_thumb,
       ScrollbarOrientation orientation,
       bool is_left_side_vertical_scrollbar,
       bool is_overlay,
-      int scrolling_layer_id);
+      int scrolling_layer_id,
+      ElementId scrolling_element_id = ElementId());
   int update_count() const { return update_count_; }
   void reset_update_count() { update_count_ = 0; }
 
@@ -56,7 +58,8 @@ class FakePaintedScrollbarLayer : public PaintedScrollbarLayer {
 
  private:
   FakePaintedScrollbarLayer(FakeScrollbar* fake_scrollbar,
-                            int scrolling_layer_id);
+                            int scrolling_layer_id,
+                            ElementId scrolling_element_id);
   ~FakePaintedScrollbarLayer() override;
 
   int update_count_;