Implement overlay scrollbar fade out for non-composited scrollers.

third_party/WebKit/Source/platform/mac/ScrollAnimatorMac.h

 /*
  * Copyright (C) 2010, 2011 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 99 matching lines @@
   void didEndScrollGesture() const;
   void mayBeginScrollGesture() const;
 
   void finishCurrentScrollAnimations() override;
 
   void didAddVerticalScrollbar(Scrollbar&) override;
   void willRemoveVerticalScrollbar(Scrollbar&) override;
   void didAddHorizontalScrollbar(Scrollbar&) override;
   void willRemoveHorizontalScrollbar(Scrollbar&) override;
 
-  bool shouldScrollbarParticipateInHitTesting(Scrollbar&) override;
-
   void notifyContentAreaScrolled(const ScrollOffset& delta) override;
 
   bool setScrollbarsVisibleForTesting(bool) override;
 
   ScrollOffset adjustScrollOffsetIfNecessary(const ScrollOffset&) const;
 
   void immediateScrollTo(const ScrollOffset&);
 
   bool m_haveScrolledSincePageLoad;
   bool m_needsScrollerStyleUpdate;
   IntRect m_visibleScrollerThumbRect;
 };
 
 }  // namespace blink
 
 #endif  // ScrollAnimatorMac_h