Simplify and add caching for accessible bounding box calculation.

Source/core/accessibility/AccessibilityScrollbar.h

 /*
  * Copyright (C) 2010 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
@@ skipping 30 matching lines @@
 
     Scrollbar* scrollbar() const { return m_scrollbar.get(); }
     
 private:
     explicit AccessibilityScrollbar(Scrollbar*);
 
     virtual bool canSetValueAttribute() const { return true; }
     virtual bool canSetNumericValue() const { return true; }
 
     virtual bool isAccessibilityScrollbar() const { return true; }
-    virtual LayoutRect elementRect() const;
+    virtual LayoutRect elementRect();
     
     virtual AccessibilityRole roleValue() const { return ScrollBarRole; }
-    virtual AccessibilityOrientation orientation() const;
+    virtual AccessibilityOrientation orientation();
     virtual Document* document() const;
     virtual bool isEnabled() const;
     
     // Assumes float [0..1]
     virtual void setValue(float);
     virtual float valueForRange() const;
 
     RefPtr<Scrollbar> m_scrollbar;
 };
 
 } // namespace WebCore
 
 #endif // AccessibilityScrollbar_h