Reviewed by Chris Fleizach.

        ARIA Grid tables should return GridRole in roleValue() method
        https://bugs.webkit.org/show_bug.cgi?id=57614

        This change does not need any test since it doesn't change anything
        from the point of view of the consumers (Assistive Technoglogies).
        It's just an internal change to simplify identifying HTML and ARIA
        tables by calling to the AccessibilityObject::roleValue method.

        * accessibility/AccessibilityARIAGrid.h:
        * accessibility/AccessibilityARIAGrid.cpp:
        (WebCore::AccessibilityARIAGrid::roleValue): Return GridRole.

        * accessibility/gtk/AccessibilityObjectWrapperAtk.cpp:
        (atkRole): Removed unneeded comment.

        * accessibility/mac/AccessibilityObjectWrapper.mm: Map GridRole to
        NSAccessibilityTableRole, to keep the same behaviour.

git-svn-id: http://svn.webkit.org/repository/webkit/trunk@82687 268f45cc-cd09-0410-ab3c-d52691b4dbfc