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..7ac4bac0327805eef4ce70d4fb9052df1b81796a 100644
--- a/cc/input/scrollbar_animation_controller.h
+++ b/cc/input/scrollbar_animation_controller.h
@@ -65,8 +65,11 @@ class CC_EXPORT ScrollbarAnimationController {
 
   bool Animate(base::TimeTicks now);
 
+  // WillUpdateScroll expects to be called even if the scroll position won't
+  // change as a result of the scroll.
+  void WillUpdateScroll();
+  void DidResize();

[weiliangc, 2017/03/01 19:24:12]

Could you also comment on when DidResize is supposed to be called? (I'm assuming
whenever scroll bar or scroll area changed size?)

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