Handle tab key presses in the DOM.

        * dom/EventTargetNode.cpp:
        (WebCore::EventTargetNode::defaultEventHandler): Add tab key handling.
        * page/FrameView.cpp:
        (WebCore::FrameView::advanceFocus): Rename and reimplement
        focusNextPrevNode. Now we just call Element::focus().
        * page/FrameView.h: Rename focusNextPrevNode to advanceFocus and make
        it public.



git-svn-id: http://svn.webkit.org/repository/webkit/trunk@16741 268f45cc-cd09-0410-ab3c-d52691b4dbfc