Prevent overlay scrollbars expand or hover together

cc/input/scrollbar_animation_controller_linear_fade.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_LINEAR_FADE_H_
 #define CC_INPUT_SCROLLBAR_ANIMATION_CONTROLLER_LINEAR_FADE_H_
 
 #include <memory>
 
 #include "base/macros.h"
@@ skipping 22 matching lines @@
       ScrollbarAnimationControllerClient* client,
       base::TimeDelta delay_before_starting,
       base::TimeDelta resize_delay_before_starting,
       base::TimeDelta duration);
 
   void RunAnimationFrame(float progress) override;
   const base::TimeDelta& Duration() override;
 
  private:
   float OpacityAtTime(base::TimeTicks now) const;
-  void ApplyOpacityToScrollbars(float opacity);
+  void ApplyOpacityToScrollbars(float opacity) override;
 
   base::TimeDelta duration_;
 
   DISALLOW_COPY_AND_ASSIGN(ScrollbarAnimationControllerLinearFade);
 };
 
 }  // namespace cc
 
 #endif  // CC_INPUT_SCROLLBAR_ANIMATION_CONTROLLER_LINEAR_FADE_H_