Disable overlay scrollbars in Blink when hidden by the compositor.

cc/input/scrollbar_animation_controller_thinning.h

 // Copyright 2013 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_THINNING_H_
 #define CC_INPUT_SCROLLBAR_ANIMATION_CONTROLLER_THINNING_H_
 
 #include <memory>
 
 #include "base/macros.h"
@@ skipping 23 matching lines @@
   }
   bool mouse_is_over_scrollbar() const { return mouse_is_over_scrollbar_; }
   bool mouse_is_near_scrollbar() const { return mouse_is_near_scrollbar_; }
 
   void DidScrollUpdate(bool on_resize) override;
 
   void DidMouseDown() override;
   void DidMouseUp() override;
   void DidMouseLeave() override;
   void DidMouseMoveNear(float distance) override;
+  bool ScrollbarsHidden() const override;
 
  protected:
   ScrollbarAnimationControllerThinning(
       int scroll_layer_id,
       ScrollbarAnimationControllerClient* client,
       base::TimeDelta delay_before_starting,
       base::TimeDelta resize_delay_before_starting,
       base::TimeDelta fade_duration,
       base::TimeDelta thinning_duration);
 
@@ skipping 29 matching lines @@
   base::TimeDelta thinning_duration_;
 
   AnimatingProperty current_animating_property_;
 
   DISALLOW_COPY_AND_ASSIGN(ScrollbarAnimationControllerThinning);
 };
 
 }  // namespace cc
 
 #endif  // CC_INPUT_SCROLLBAR_ANIMATION_CONTROLLER_THINNING_H_