Implement overlay scrollbar fade out for non-composited scrollers.

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

 /*
  * 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 934 matching lines @@
     return;
 
   ASSERT(m_horizontalScrollbarPainterDelegate);
   [m_horizontalScrollbarPainterDelegate.get() invalidate];
   m_horizontalScrollbarPainterDelegate = nullptr;
 
   [painter setDelegate:nil];
   [m_scrollbarPainterController.get() setHorizontalScrollerImp:nil];
 }
 
-bool ScrollAnimatorMac::shouldScrollbarParticipateInHitTesting(
-    Scrollbar& scrollbar) {
-  // Non-overlay scrollbars should always participate in hit testing.
-  if (ScrollbarThemeMac::recommendedScrollerStyle() != NSScrollerStyleOverlay)
-    return true;
-
-  // Overlay scrollbars should participate in hit testing whenever they are at
-  // all visible.
-  ScrollbarPainter painter = scrollbarPainterForScrollbar(scrollbar);
-  if (!painter)
-    return false;
-  return [painter knobAlpha] > 0;
-}
-
 void ScrollAnimatorMac::notifyContentAreaScrolled(const ScrollOffset& delta) {
   // This function is called when a page is going into the page cache, but the
   // page
   // isn't really scrolling in that case. We should only pass the message on to
   // the
   // ScrollbarPainterController when we're really scrolling on an active page.
   if (getScrollableArea()->scrollbarsCanBeActive())
     sendContentAreaScrolledSoon(delta);
 }
 
@@ skipping 153 matching lines @@
     rectInViewCoordinates =
         verticalScrollbar->convertToContainingWidget(scrollerThumb);
 
   if (rectInViewCoordinates == m_visibleScrollerThumbRect)
     return;
 
   m_visibleScrollerThumbRect = rectInViewCoordinates;
 }
 
 }  // namespace blink