Implement overlay scrollbar fade out for non-composited scrollers.

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

 /*
  * Copyright (C) 2008, 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 74 matching lines @@
   [_scrollbarPainter.get() removeObserver:self forKeyPath:@"knobAlpha"];
   [super dealloc];
 }
 
 - (void)observeValueForKeyPath:(NSString*)keyPath
                       ofObject:(id)object
                         change:(NSDictionary*)change
                        context:(void*)context {
   if ([keyPath isEqualToString:@"knobAlpha"]) {
     BOOL visible = [_scrollbarPainter.get() knobAlpha] > 0;
-    if (_visible != visible) {
-      _visible = visible;
-      _scrollbar->visibilityChanged();
-    }
+    _scrollbar->setScrollbarsHidden(!visible);

[dtapuska, 2016/11/02 16:53:21]

Can we remove the _visible member now?

[bokan, 2016/11/02 22:41:57]

Done.

   }
 }
 
 @end
 
 namespace blink {
 
 typedef HashSet<ScrollbarThemeClient*> ScrollbarSet;
 
 static ScrollbarSet& scrollbarSet() {
@@ skipping 358 matching lines @@
 }
 
 // static
 NSScrollerStyle ScrollbarThemeMac::recommendedScrollerStyle() {
   if (RuntimeEnabledFeatures::overlayScrollbarsEnabled())
     return NSScrollerStyleOverlay;
   return gPreferredScrollerStyle;
 }
 
 }  // namespace blink