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 02f8e97d2cbc049b1691203b47583a481dab10ec..262354386331b306337d8587da29aadff14ffb26 100644
--- a/cc/input/scrollbar_animation_controller.h
+++ b/cc/input/scrollbar_animation_controller.h
@@ -65,8 +65,19 @@ 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. Only effect Aura Overlay Scrollbar.
+  void WillUpdateScroll();
+
+  // DidScrollUpdate expects to be called only if the scroll position change.
+  // Effect both Android and Aura Overlay Scrollbar.
+  void DidScrollUpdate();
+
+  // DidResize expects to be called when clip layer size changed or scroll layer
+  // size changed.
+  void DidResize();
+
   void DidScrollBegin();
-  void DidScrollUpdate(bool on_resize);
   void DidScrollEnd();
 
   void DidMouseDown();
@@ -143,6 +154,7 @@ class CC_EXPORT ScrollbarAnimationController {
   float opacity_;
   base::TimeDelta fade_out_duration_;
 
+  const bool show_scrollbars_on_scroll_gesture_;
   const bool need_thinning_animation_;
   std::unique_ptr<SingleScrollbarAnimationControllerThinning>
       vertical_controller_;