- fixed <rdar://problem/4566087> REGRESSION (420+): Crash occurs while completing a find again query at 
        http://www.apple.com/ (RenderObject::repaint(bool))
        
        This wasn't a new problem in the code, but the recent use of DocumentMarkers for find results brought
        it to the surface.

        * dom/Document.h:
        Use a RefPtr<Node> instead of a Node* in MarkerMap, so the node will be retained

        * dom/Document.cpp:
        (WebCore::Document::removeMarkers):
        Add a .get() to compensate for using RefPtr. Also, only repaint if this node actually had a marker
        removed.
        (WebCore::Document::repaintMarkers):
        Add a .get() to compensate for using RefPtr.



git-svn-id: http://svn.webkit.org/repository/webkit/trunk@14748 268f45cc-cd09-0410-ab3c-d52691b4dbfc