Implement overlay scrollbar fade out for non-composited scrollers.

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

 /*
  * Copyright (C) 2008 Apple 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 85 matching lines @@
                                           const PlatformMouseEvent&);
   virtual bool shouldDragDocumentInsteadOfThumb(const ScrollbarThemeClient&,
                                                 const PlatformMouseEvent&) {
     return false;
   }
 
   // The position of the thumb relative to the track.
   int thumbPosition(const ScrollbarThemeClient& scrollbar) {
     return thumbPosition(scrollbar, scrollbar.currentPos());
   }
+  virtual double overlayScrollbarFadeOutDelaySeconds() const;
+  virtual double overlayScrollbarFadeOutDurationSeconds() const;
   // The position the thumb would have, relative to the track, at the specified
   // scroll position.
   virtual int thumbPosition(const ScrollbarThemeClient&, float scrollPosition);
   // The length of the thumb along the axis of the scrollbar.
   virtual int thumbLength(const ScrollbarThemeClient&);
   // The position of the track relative to the scrollbar.
   virtual int trackPosition(const ScrollbarThemeClient&);
   // The length of the track along the axis of the scrollbar.
   virtual int trackLength(const ScrollbarThemeClient&);
   // The opacity to be applied to the thumb.
@@ skipping 62 matching lines @@
   static DisplayItem::Type trackPiecePartToDisplayItemType(ScrollbarPart);
 
  private:
   static ScrollbarTheme&
   nativeTheme();  // Must be implemented to return the correct theme subclass.
   static bool gMockScrollbarsEnabled;
 };
 
 }  // namespace blink
 #endif