Reviewed by Eric Seidel.

        Bug 27956 - AX: roleValue should be cached for performance
        https://bugs.webkit.org/show_bug.cgi?id=27956

        Caches the role value of an accessibility object instead of calculating it everytime.
        The changes gained a 5% speedup using VoiceOver to navigate a webpage.

        * accessibility/AXObjectCache.cpp:
        (WebCore::AXObjectCache::handleAriaRoleChanged):

        Use generic role setter instead of specific ARIA role setter.

        * accessibility/AccessibilityObject.cpp:
        (WebCore::AccessibilityObject::AccessibilityObject):
        * accessibility/AccessibilityObject.h:
        (WebCore::AccessibilityObject::headingLevel):

        Make headingLevel into instance method so that it can *safely* be called in the constructor.

        (WebCore::AccessibilityObject::setRoleValue):
        (WebCore::AccessibilityObject::roleValue):
        * accessibility/AccessibilityRenderObject.cpp:
        (WebCore::AccessibilityRenderObject::AccessibilityRenderObject):
        (WebCore::AccessibilityRenderObject::headingLevel):
        (WebCore::AccessibilityRenderObject::intValue):
        (WebCore::AccessibilityRenderObject::updateAccessibilityRole):
        (WebCore::AccessibilityRenderObject::determineAccessibilityRole):
        * accessibility/AccessibilityRenderObject.h:
        * accessibility/mac/AccessibilityObjectWrapper.mm:
        (AXAttributeStringSetHeadingLevel):



git-svn-id: http://svn.webkit.org/repository/webkit/trunk@46878 268f45cc-cd09-0410-ab3c-d52691b4dbfc