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

Source/core/xml/XPathStep.cpp

 /*
  * Copyright (C) 2005 Frerich Raabe <raabe@kde.org>
  * Copyright (C) 2006, 2009 Apple Inc. All rights reserved.
  * 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
@@ skipping 174 matching lines @@
             ASSERT(axis != Step::NamespaceAxis);
 
             // For other axes, the principal node type is element.
             ASSERT(primaryNodeType(axis) == Node::ELEMENT_NODE);
             if (node->nodeType() != Node::ELEMENT_NODE)
                 return false;
 
             if (name == starAtom)
                 return namespaceURI.isEmpty() || namespaceURI == node->namespaceURI();
 
-            if (node->document()->isHTMLDocument()) {
+            if (node->document().isHTMLDocument()) {
                 if (node->isHTMLElement()) {
                     // Paths without namespaces should match HTML elements in HTML documents despite those having an XHTML namespace. Names are compared case-insensitively.
                     return equalIgnoringCase(toElement(node)->localName(), name) && (namespaceURI.isNull() || namespaceURI == node->namespaceURI());
                 }
                 // An expression without any prefix shouldn't match no-namespace nodes (because HTML5 says so).
                 return toElement(node)->hasLocalName(name) && namespaceURI == node->namespaceURI() && !namespaceURI.isNull();
             }
             return toElement(node)->hasLocalName(name) && namespaceURI == node->namespaceURI();
         }
     }
@@ skipping 181 matching lines @@
             nodes.markSorted(false);
             return;
         }
     }
     ASSERT_NOT_REACHED();
 }
 
 
 }
 }