Have Node::document() return a reference instead of a pointer

Source/core/xml/XPathNodeSet.cpp

 /*
  * Copyright (C) 2007 Alexey Proskuryakov <ap@webkit.org>
  *
  * Redistribution and use in source and binary forms, with or without
  * modification, are permitted provided that the following conditions
  * are met:
  *
  * 1. Redistributions of source code must retain the above copyright
  *    notice, this list of conditions and the following disclaimer.
  * 2. Redistributions in binary form must reproduce the above copyright
@@ skipping 164 matching lines @@
         sortedNodes.append(parentMatrix[i][0]);
 
     const_cast<Vector<RefPtr<Node> >&>(m_nodes).swap(sortedNodes);
 }
 
 static Node* findRootNode(Node* node)
 {
     if (node->isAttributeNode())
         node = toAttr(node)->ownerElement();
     if (node->inDocument())
-        node = node->document();
+        node = &node->document();
     else {
         while (Node* parent = node->parentNode())
             node = parent;
     }
     return node;
 }
 
 void NodeSet::traversalSort() const
 {
     HashSet<Node*> nodes;
@@ skipping 60 matching lines @@
 Node* NodeSet::anyNode() const
 {
     if (isEmpty())
         return 0;
 
     return m_nodes.at(0).get();
 }
 
 }
 }