Prevent create ScrollbarAnimationController for Mac Overlay Scrollbar

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 7becd72c69e4d89ff8984e80c3357b2b2cd72fca..f34fc2cbd4f94641ec94aeea58b5f78126165c6c 100644
--- a/cc/layers/scrollbar_layer_impl_base.h
+++ b/cc/layers/scrollbar_layer_impl_base.h
@@ -61,6 +61,8 @@ class CC_EXPORT ScrollbarLayerImplBase : public LayerImpl {
   // on overlay scrollbar layers.
   void SetOverlayScrollbarLayerOpacityAnimated(float opacity);
 
+  virtual bool IsSolidColorScrollbarLayerImpl() const;

[aelias_OOO_until_Jul13, 2017/06/02 22:02:23]

How about instead making scrollbars return one of the animator enum types to
specify which one they're compatible with?

[chaopeng, 2017/06/05 14:52:39]

Yes, this would be better.

+
  protected:
   ScrollbarLayerImplBase(LayerTreeImpl* tree_impl,
                          int id,