Show Overlay Scrollbar when GestureScrollUpdate

cc/input/scrollbar_animation_controller.h

 // Copyright 2012 The Chromium Authors. All rights reserved.
 // Use of this source code is governed by a BSD-style license that can be
 // found in the LICENSE file.
 
 #ifndef CC_INPUT_SCROLLBAR_ANIMATION_CONTROLLER_H_
 #define CC_INPUT_SCROLLBAR_ANIMATION_CONTROLLER_H_
 
 #include "base/cancelable_callback.h"
 #include "base/memory/weak_ptr.h"
 #include "base/time/time.h"
@@ skipping 47 matching lines @@
       base::TimeDelta fade_out_resize_delay,
       base::TimeDelta fade_out_duration,
       base::TimeDelta thinning_duration);
 
   ~ScrollbarAnimationController();
 
   bool ScrollbarsHidden() const;
 
   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();
   void DidMouseUp();
   void DidMouseLeave();
   void DidMouseMoveNear(ScrollbarOrientation, float);
 
   bool mouse_is_over_scrollbar(ScrollbarOrientation orientation) const;
   bool mouse_is_near_scrollbar(ScrollbarOrientation orientation) const;
   bool mouse_is_near_any_scrollbar() const;
@@ skipping 66 matching lines @@
       vertical_controller_;
   std::unique_ptr<SingleScrollbarAnimationControllerThinning>
       horizontal_controller_;
 
   base::WeakPtrFactory<ScrollbarAnimationController> weak_factory_;
 };
 
 }  // namespace cc
 
 #endif  // CC_INPUT_SCROLLBAR_ANIMATION_CONTROLLER_H_