Simplify and add caching for accessible bounding box calculation.

Source/core/accessibility/AccessibilitySpinButton.cpp

Index: Source/core/accessibility/AccessibilitySpinButton.cpp
diff --git a/Source/core/accessibility/AccessibilitySpinButton.cpp b/Source/core/accessibility/AccessibilitySpinButton.cpp
index 95412d9034fe92286d4ffd84023a9544764b42a0..d84b1507645289f997f31daa142965a69b75f5e4 100644
--- a/Source/core/accessibility/AccessibilitySpinButton.cpp
+++ b/Source/core/accessibility/AccessibilitySpinButton.cpp
@@ -65,7 +65,7 @@ AccessibilityObject* AccessibilitySpinButton::decrementButton()
     return m_children[1].get();    
 }
     
-LayoutRect AccessibilitySpinButton::elementRect() const
+LayoutRect AccessibilitySpinButton::elementRect()
 {
     ASSERT(m_spinButtonElement);
     
@@ -114,7 +114,7 @@ PassRefPtr<AccessibilitySpinButtonPart> AccessibilitySpinButtonPart::create()
     return adoptRef(new AccessibilitySpinButtonPart);
 }
 
-LayoutRect AccessibilitySpinButtonPart::elementRect() const
+LayoutRect AccessibilitySpinButtonPart::elementRect()
 {
     // FIXME: This logic should exist in the render tree or elsewhere, but there is no
     // relationship that exists that can be queried.