Reviewed by Darin.

        Return an AccessibleDocument when our root accessible object is queried
        for.

        * AccessibleDocument.cpp: Added; represents an AccessibilityObject that
        itself represents a Document.
        (AccessibleDocument::AccessibleDocument): Initialize AccessibleBase with
        the AccessibilityObject for this Document.
        * AccessibleDocument.h: Added.
        * WebFrame.cpp:
        (WebFrame::accessible): If m_accessible is 0, or if Document in this
        frame does not match the Document in the cached m_accessible object,
        create a new AccessibleDocument for this frame's Document.
        * WebFrame.h: Swap AccessibleBase.h for AccessibleDocument.h
        * WebKit.vcproj/WebKit.vcproj: Add AccessibleDocument.{h,cpp}


git-svn-id: http://svn.webkit.org/repository/webkit/trunk@32250 268f45cc-cd09-0410-ab3c-d52691b4dbfc