Simplify and add caching for accessible bounding box calculation.

Source/core/accessibility/AccessibilitySlider.cpp

Index: Source/core/accessibility/AccessibilitySlider.cpp
diff --git a/Source/core/accessibility/AccessibilitySlider.cpp b/Source/core/accessibility/AccessibilitySlider.cpp
index f29104df6fb16f62a81aea0875504907369aaee2..6d0be4bfc08bcc0c59517883fbcd8d29445b2f04 100644
--- a/Source/core/accessibility/AccessibilitySlider.cpp
+++ b/Source/core/accessibility/AccessibilitySlider.cpp
@@ -47,7 +47,7 @@ PassRefPtr<AccessibilitySlider> AccessibilitySlider::create(RenderObject* render
     return adoptRef(new AccessibilitySlider(renderer));
 }
 
-AccessibilityOrientation AccessibilitySlider::orientation() const
+AccessibilityOrientation AccessibilitySlider::orientation()
 {
     // Default to horizontal in the unknown case.
     if (!m_renderer)
@@ -99,7 +99,7 @@ const AtomicString& AccessibilitySlider::getAttribute(const QualifiedName& attri
     return element()->getAttribute(attribute);
 }
     
-AccessibilityObject* AccessibilitySlider::elementAccessibilityHitTest(const IntPoint& point) const
+AccessibilityObject* AccessibilitySlider::elementAccessibilityHitTest(const IntPoint& point)
 {
     if (m_children.size()) {
         ASSERT(m_children.size() == 1);
@@ -153,7 +153,7 @@ PassRefPtr<AccessibilitySliderThumb> AccessibilitySliderThumb::create()
     return adoptRef(new AccessibilitySliderThumb());
 }
     
-LayoutRect AccessibilitySliderThumb::elementRect() const
+LayoutRect AccessibilitySliderThumb::elementRect()
 {
     if (!m_parent)
         return LayoutRect();