cc: LayerTreeHostImpl uses element id to tick animations

cc/layers/scrollbar_layer_impl_base.h

Index: cc/layers/scrollbar_layer_impl_base.h
diff --git a/cc/layers/scrollbar_layer_impl_base.h b/cc/layers/scrollbar_layer_impl_base.h
index 05c289128be39f944652f8939249c7c77c8df54a..0b9b403950bc1f044af3c8ba114e9ef754e271d2 100644
--- a/cc/layers/scrollbar_layer_impl_base.h
+++ b/cc/layers/scrollbar_layer_impl_base.h
@@ -56,6 +56,10 @@ class CC_EXPORT ScrollbarLayerImplBase : public LayerImpl {
 
   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,