Implement overlay scrollbar fade out for non-composited scrollers.

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

Index: third_party/WebKit/Source/platform/mac/ScrollAnimatorMac.mm
diff --git a/third_party/WebKit/Source/platform/mac/ScrollAnimatorMac.mm b/third_party/WebKit/Source/platform/mac/ScrollAnimatorMac.mm
index 79356732195909885ce20fbac12ebf74221af731..a49351808fc115c54467c0f1efafa3d14ffc4027 100644
--- a/third_party/WebKit/Source/platform/mac/ScrollAnimatorMac.mm
+++ b/third_party/WebKit/Source/platform/mac/ScrollAnimatorMac.mm
@@ -952,20 +952,6 @@ void ScrollAnimatorMac::willRemoveHorizontalScrollbar(Scrollbar& scrollbar) {
   [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