Implement overlay scrollbar fade out for non-composited scrollers.

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

 /*
  * Copyright (C) 2011 Google 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
@@ skipping 31 matching lines @@
                         HitTestBehavior);
   ScrollbarThemeOverlay(int thumbThickness,
                         int scrollbarMargin,
                         HitTestBehavior,
                         Color);
   ~ScrollbarThemeOverlay() override {}
 
   int scrollbarThickness(ScrollbarControlSize) override;
   int scrollbarMargin() const override;
   bool usesOverlayScrollbars() const override;
+  double overlayScrollbarFadeOutDelaySeconds() const override;
+  double overlayScrollbarFadeOutDurationSeconds() const override;
 
   int thumbPosition(const ScrollbarThemeClient&, float scrollPosition) override;
   int thumbLength(const ScrollbarThemeClient&) override;
 
   bool hasButtons(const ScrollbarThemeClient&) override { return false; }
   bool hasThumb(const ScrollbarThemeClient&) override;
 
   IntRect backButtonRect(const ScrollbarThemeClient&,
                          ScrollbarPart,
                          bool painting = false) override;
@@ skipping 14 matching lines @@
   int m_thumbThickness;
   int m_scrollbarMargin;
   HitTestBehavior m_allowHitTest;
   Color m_color;
   const bool m_useSolidColor;
 };
 
 }  // namespace blink
 
 #endif