Implement overlay scrollbar fade out for non-composited scrollers.

third_party/WebKit/Source/platform/scroll/ScrollbarThemeMac.mm

Index: third_party/WebKit/Source/platform/scroll/ScrollbarThemeMac.mm
diff --git a/third_party/WebKit/Source/platform/scroll/ScrollbarThemeMac.mm b/third_party/WebKit/Source/platform/scroll/ScrollbarThemeMac.mm
index 60d97556f4c6f11a0934be919c53a2d87cbd020b..861c471961dc43bb9cd44e920ab84a92c2a0bd95 100644
--- a/third_party/WebKit/Source/platform/scroll/ScrollbarThemeMac.mm
+++ b/third_party/WebKit/Source/platform/scroll/ScrollbarThemeMac.mm
@@ -56,7 +56,6 @@ using namespace blink;
 @interface BlinkScrollbarObserver : NSObject {
   blink::ScrollbarThemeClient* _scrollbar;
   RetainPtr<ScrollbarPainter> _scrollbarPainter;
-  BOOL _visible;
 }
 - (id)initWithScrollbar:(blink::ScrollbarThemeClient*)scrollbar
                 painter:(const RetainPtr<ScrollbarPainter>&)painter;
@@ -92,10 +91,7 @@ using namespace blink;
                        context:(void*)context {
   if ([keyPath isEqualToString:@"knobAlpha"]) {
     BOOL visible = [_scrollbarPainter.get() knobAlpha] > 0;
-    if (_visible != visible) {
-      _visible = visible;
-      _scrollbar->visibilityChanged();
-    }
+    _scrollbar->setScrollbarsHidden(!visible);
   }
 }