Implement overlay scrollbar fade out for non-composited scrollers.

third_party/WebKit/Source/platform/scroll/Scrollbar.h

 /*
  * Copyright (C) 2004, 2006 Apple Computer, Inc.  All rights reserved.
  *
  * Redistribution and use in source and binary forms, with or without
  * modification, are permitted provided that the following conditions
  * are met:
  * 1. Redistributions of source code must retain the above copyright
  *    notice, this list of conditions and the following disclaimer.
  * 2. Redistributions in binary form must reproduce the above copyright
  *    notice, this list of conditions and the following disclaimer in the
@@ skipping 86 matching lines @@
   float currentPos() const override { return m_currentPos; }
   int visibleSize() const override { return m_visibleSize; }
   int totalSize() const override { return m_totalSize; }
   int maximum() const override { return m_totalSize - m_visibleSize; }
   ScrollbarControlSize controlSize() const override { return m_controlSize; }
 
   ScrollbarPart pressedPart() const override { return m_pressedPart; }
   ScrollbarPart hoveredPart() const override { return m_hoveredPart; }
 
   void styleChanged() override {}
-  void visibilityChanged() override;
+  void setScrollbarsHidden(bool) override;
   bool enabled() const override { return m_enabled; }
   void setEnabled(bool) override;
 
   int scrollbarThickness() const;
 
   // Called by the ScrollableArea when the scroll offset changes.
   // Will trigger paint invalidation if required.
   void offsetDidChange();
 
   void disconnectFromScrollableArea();
@@ skipping 137 matching lines @@
 
 DEFINE_TYPE_CASTS(Scrollbar,
                   Widget,
                   widget,
                   widget->isScrollbar(),
                   widget.isScrollbar());
 
 }  // namespace blink
 
 #endif  // Scrollbar_h