Simplify and add caching for accessible bounding box calculation.

Source/core/accessibility/AccessibilityListBox.cpp

Index: Source/core/accessibility/AccessibilityListBox.cpp
diff --git a/Source/core/accessibility/AccessibilityListBox.cpp b/Source/core/accessibility/AccessibilityListBox.cpp
index 718a27eba0e2c8878790b3cf0c3968ab5b8e78ae..d03f2adef9c5b6bc995dfbb73deba33eb6786fb3 100644
--- a/Source/core/accessibility/AccessibilityListBox.cpp
+++ b/Source/core/accessibility/AccessibilityListBox.cpp
@@ -149,7 +149,7 @@ AccessibilityObject* AccessibilityListBox::listBoxOptionAccessibilityObject(HTML
     return listBoxObject;
 }
     
-AccessibilityObject* AccessibilityListBox::elementAccessibilityHitTest(const IntPoint& point) const
+AccessibilityObject* AccessibilityListBox::elementAccessibilityHitTest(const IntPoint& point)
 {
     // the internal HTMLSelectElement methods for returning a listbox option at a point
     // ignore optgroup elements.
@@ -160,7 +160,7 @@ AccessibilityObject* AccessibilityListBox::elementAccessibilityHitTest(const Int
     if (!node)
         return 0;
     
-    LayoutRect parentRect = boundingBoxRect();
+    LayoutRect parentRect = elementRect();
     
     AccessibilityObject* listBoxOption = 0;
     unsigned length = m_children.size();