Simplify and add caching for accessible bounding box calculation.

Source/core/accessibility/AccessibilityScrollbar.cpp

Index: Source/core/accessibility/AccessibilityScrollbar.cpp
diff --git a/Source/core/accessibility/AccessibilityScrollbar.cpp b/Source/core/accessibility/AccessibilityScrollbar.cpp
index 062236d34e665d88979aef1e06c2526c42cfdde7..8904753dc608ab11614101265804c6a57becf61c 100644
--- a/Source/core/accessibility/AccessibilityScrollbar.cpp
+++ b/Source/core/accessibility/AccessibilityScrollbar.cpp
@@ -44,7 +44,7 @@ PassRefPtr<AccessibilityScrollbar> AccessibilityScrollbar::create(Scrollbar* scr
     return adoptRef(new AccessibilityScrollbar(scrollbar));
 }
     
-LayoutRect AccessibilityScrollbar::elementRect() const
+LayoutRect AccessibilityScrollbar::elementRect()
 {
     if (!m_scrollbar)
         return LayoutRect();
@@ -60,7 +60,7 @@ Document* AccessibilityScrollbar::document() const
     return parent->document();
 }
 
-AccessibilityOrientation AccessibilityScrollbar::orientation() const
+AccessibilityOrientation AccessibilityScrollbar::orientation()
 {
     if (!m_scrollbar)
         return AccessibilityOrientationHorizontal;