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

Source/core/xml/XPathExpression.cpp

 /*
  * Copyright (C) 2005 Frerich Raabe <raabe@kde.org>
  * Copyright (C) 2006, 2009 Apple Inc. All rights reserved.
  *
  * 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.
@@ skipping 51 matching lines @@
     if (!isValidContextNode(contextNode)) {
         es.throwDOMException(NotSupportedError);
         return 0;
     }
 
     EvaluationContext& evaluationContext = Expression::evaluationContext();
     evaluationContext.node = contextNode;
     evaluationContext.size = 1;
     evaluationContext.position = 1;
     evaluationContext.hadTypeConversionError = false;
-    RefPtr<XPathResult> result = XPathResult::create(contextNode->document(), m_topExpression->evaluate());
+    RefPtr<XPathResult> result = XPathResult::create(&contextNode->document(), m_topExpression->evaluate());
     evaluationContext.node = 0; // Do not hold a reference to the context node, as this may prevent the whole document from being destroyed in time.
 
     if (evaluationContext.hadTypeConversionError) {
         // It is not specified what to do if type conversion fails while evaluating an expression.
         es.throwDOMException(SyntaxError);
         return 0;
     }
 
     if (type != XPathResult::ANY_TYPE) {
         result->convertTo(type, es);
         if (es.hadException())
             return 0;
     }
 
     return result;
 }
 
 }