Reland of Fix several corner case issues of scrollbar paint invalidation

third_party/WebKit/Source/platform/scroll/ScrollableArea.cpp

 /*
  * Copyright (c) 2010, Google Inc. All rights reserved.
  * 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:
  *
  *     * Redistributions of source code must retain the above copyright
  * notice, this list of conditions and the following disclaimer.
@@ skipping 388 matching lines @@
 
     if (Scrollbar* scrollbar = verticalScrollbar()) {
         ScrollbarTheme::theme()->updateScrollbarOverlayStyle(scrollbar);
         setScrollbarNeedsPaintInvalidation(scrollbar);
     }
 }
 
 void ScrollableArea::setScrollbarNeedsPaintInvalidation(Scrollbar* scrollbar)
 {
     if (scrollbar == horizontalScrollbar()) {
-        if (GraphicsLayer* graphicsLayer = layerForHorizontalScrollbar()) {
-            graphicsLayer->setNeedsDisplay();
-            graphicsLayer->setContentsNeedsDisplay();
-            return;
-        }
         m_horizontalScrollbarNeedsPaintInvalidation = true;
         scrollControlWasSetNeedsPaintInvalidation();
         return;
     }
     if (scrollbar == verticalScrollbar()) {
-        if (GraphicsLayer* graphicsLayer = layerForVerticalScrollbar()) {
-            graphicsLayer->setNeedsDisplay();
-            graphicsLayer->setContentsNeedsDisplay();
-            return;
-        }
         m_verticalScrollbarNeedsPaintInvalidation = true;
         scrollControlWasSetNeedsPaintInvalidation();
         return;
     }
     // Otherwise the scrollbar is just created and has not been set as either
     // horizontalScrollbar() or verticalScrollbar().
 }
 
 void ScrollableArea::setScrollCornerNeedsPaintInvalidation()
 {
@@ skipping 149 matching lines @@
 
 }
 
 DEFINE_TRACE(ScrollableArea)
 {
     visitor->trace(m_scrollAnimator);
     visitor->trace(m_programmaticScrollAnimator);
 }
 
 } // namespace blink