Simplify and add caching for accessible bounding box calculation.

Source/core/accessibility/AccessibilityMenuListPopup.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
  *    notice, this list of conditions and the following disclaimer in the
@@ skipping 32 matching lines @@
     virtual bool isOffScreen() const;
 
     void didUpdateActiveOption(int optionIndex);
 
 
 private:
     AccessibilityMenuListPopup();
 
     virtual bool isMenuListPopup() const { return true; }
 
-    virtual LayoutRect elementRect() const { return LayoutRect(); }
+    virtual LayoutRect elementRect() { return LayoutRect(); }
     virtual AccessibilityRole roleValue() const { return MenuListPopupRole; }
 
     virtual bool isVisible() const;
     virtual bool press() const;
     virtual void addChildren();
     virtual void childrenChanged();
     virtual bool computeAccessibilityIsIgnored() const;
 
     AccessibilityMenuListOption* menuListOptionAccessibilityObject(HTMLElement*) const;
 };
 
 } // namespace WebCore
 
 #endif // AccessibilityMenuListPopup_h