Reviewed by Mark Rowe.

        - fix https://bugs.webkit.org/show_bug.cgi?id=21557
          Bug 21557: REGRESSION: generated-layer-scrollbar-crash.html is crashing

        * rendering/RenderObject.cpp:
        (WebCore::RenderObject::getPseudoStyle): Keep walking up the parent chain
        until we hit an Element. Nowadays, Node::isElementNode() is just as efficient
        as RenderObject::isText(), and this guarantees that the node can be cast to
        Element*, which is done just below. The reason this is hit is that RenderLayer
        can call getPseudoStyle on a RenderView, which has the document as its
        element() -- and a document is neither a Text node nor an Element. A loose
        end is that this seems to mean you can't style the scrollbar corner or the
        resizer on a RenderView. Hyatt may want to look into that later.



git-svn-id: http://svn.webkit.org/repository/webkit/trunk@37533 268f45cc-cd09-0410-ab3c-d52691b4dbfc