Add infrastructure for partial layouts

Source/core/rendering/RenderScrollbarPart.h

 /*
  * Copyright (C) 2008 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 48 matching lines @@
 protected:
     virtual void styleWillChange(StyleDifference diff, const RenderStyle* newStyle);
     virtual void styleDidChange(StyleDifference, const RenderStyle* oldStyle);
     virtual void imageChanged(WrappedImagePtr, const IntRect* = 0);
 
 private:
     RenderScrollbarPart(RenderScrollbar*, ScrollbarPart);
 
     virtual void computePreferredLogicalWidths();
 
+    virtual bool supportsPartialLayout() const OVERRIDE { return false; }
+
     void layoutHorizontalPart();
     void layoutVerticalPart();
 
     void computeScrollbarWidth();
     void computeScrollbarHeight();
 
     RenderScrollbar* m_scrollbar;
     ScrollbarPart m_part;
 };
 
 inline RenderScrollbarPart* toRenderScrollbarPart(RenderObject* object)
 {
     ASSERT_WITH_SECURITY_IMPLICATION(!object || object->isRenderScrollbarPart());
     return static_cast<RenderScrollbarPart*>(object);
 }
 
 inline const RenderScrollbarPart* toRenderScrollbarPart(const RenderObject* object)
 {
     ASSERT_WITH_SECURITY_IMPLICATION(!object || object->isRenderScrollbarPart());
     return static_cast<const RenderScrollbarPart*>(object);
 }
 
 // This will catch anyone doing an unnecessary cast.
 void toRenderScrollbarPart(const RenderScrollbarPart*);
 
 } // namespace WebCore
 
 #endif // RenderScrollbarPart_h