Simplify and add caching for accessible bounding box calculation.

Source/core/accessibility/AccessibilitySlider.h

Index: Source/core/accessibility/AccessibilitySlider.h
diff --git a/Source/core/accessibility/AccessibilitySlider.h b/Source/core/accessibility/AccessibilitySlider.h
index d38748d31c55afed59ad76ca32045c2668e6a08d..03f89d5b015e07882f0b92c552904fd5de782446 100644
--- a/Source/core/accessibility/AccessibilitySlider.h
+++ b/Source/core/accessibility/AccessibilitySlider.h
@@ -47,7 +47,7 @@ protected:
 
 private:
     HTMLInputElement* element() const;
-    virtual AccessibilityObject* elementAccessibilityHitTest(const IntPoint&) const;
+    virtual AccessibilityObject* elementAccessibilityHitTest(const IntPoint&);
 
     virtual AccessibilityRole roleValue() const { return SliderRole; }    
     virtual bool isSlider() const { return true; }
@@ -63,7 +63,7 @@ private:
     virtual float valueForRange() const;
     virtual float maxValueForRange() const;
     virtual float minValueForRange() const;
-    virtual AccessibilityOrientation orientation() const;    
+    virtual AccessibilityOrientation orientation();
 };
 
 class AccessibilitySliderThumb : public AccessibilityMockObject {
@@ -74,7 +74,7 @@ public:
 
     virtual AccessibilityRole roleValue() const { return SliderThumbRole; }
 
-    virtual LayoutRect elementRect() const;
+    virtual LayoutRect elementRect();
 
 private:
     AccessibilitySliderThumb();