Simplify and add caching for accessible bounding box calculation.

Source/core/accessibility/AccessibilityListBoxOption.cpp

Index: Source/core/accessibility/AccessibilityListBoxOption.cpp
diff --git a/Source/core/accessibility/AccessibilityListBoxOption.cpp b/Source/core/accessibility/AccessibilityListBoxOption.cpp
index dd3dbe98f9fb67857c66605887c8521da3b740a6..fa49f22fe0e50b3310f5881e4e72a5301fbcc17c 100644
--- a/Source/core/accessibility/AccessibilityListBoxOption.cpp
+++ b/Source/core/accessibility/AccessibilityListBoxOption.cpp
@@ -92,7 +92,7 @@ bool AccessibilityListBoxOption::isSelectedOptionActive() const
     return listBoxParentNode->activeSelectionEndListIndex() == listBoxOptionIndex();
 }
 
-LayoutRect AccessibilityListBoxOption::elementRect() const
+LayoutRect AccessibilityListBoxOption::elementRect()
 {
     LayoutRect rect;
     if (!m_optionElement)
@@ -106,7 +106,7 @@ LayoutRect AccessibilityListBoxOption::elementRect() const
     if (!listBoxRenderer)
         return rect;
     
-    LayoutRect parentRect = listBoxRenderer->document()->axObjectCache()->getOrCreate(listBoxRenderer)->boundingBoxRect();
+    LayoutRect parentRect = listBoxRenderer->document()->axObjectCache()->getOrCreate(listBoxRenderer)->elementRect();
     int index = listBoxOptionIndex();
     if (index != -1)
         rect = toRenderListBox(listBoxRenderer)->itemBoundingBoxRect(parentRect.location(), index);