Show Overlay Scrollbar when GestureScrollUpdate

cc/input/scrollbar_animation_controller.h

Index: cc/input/scrollbar_animation_controller.h
diff --git a/cc/input/scrollbar_animation_controller.h b/cc/input/scrollbar_animation_controller.h
index 6838fff9b451a65bf0dc81b15075c2eeed7c2452..dcc58f494ec93df7f1d395fd106e4bd75908fd94 100644
--- a/cc/input/scrollbar_animation_controller.h
+++ b/cc/input/scrollbar_animation_controller.h
@@ -65,8 +65,9 @@ class CC_EXPORT ScrollbarAnimationController {
 
   bool Animate(base::TimeTicks now);
 
+  void WillUpdateScroll();

[bokan, 2017/02/28 14:42:27]

Please add a comment here that this expects to be called even if the scroll
position won't change as a result of the scroll.

+  void DidResize();
   void DidScrollBegin();
-  void DidScrollUpdate(bool on_resize);
   void DidScrollEnd();
 
   void DidMouseDown();