Have Node::highestAncestor() return a reference

Source/core/xml/XPathPath.cpp

 /*
  * Copyright (C) 2005 Frerich Raabe <raabe@kde.org>
  * Copyright (C) 2006, 2009 Apple Inc.
  * 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 87 matching lines @@
     // "A / by itself selects the root node of the document containing the context node."
     // In the case of a tree that is detached from the document, we violate
     // the spec and treat / as the root node of the detached tree.
     // This is for compatibility with Firefox, and also seems like a more
     // logical treatment of where you would expect the "root" to be.
     Node* context = evaluationContext.node.get();
     if (m_absolute && context->nodeType() != Node::DOCUMENT_NODE)  {
         if (context->inDocument())
             context = context->ownerDocument();
         else
-            context = context->highestAncestor();
+            context = &context->highestAncestor();
     }
 
     NodeSet nodes;
     nodes.append(context);
     evaluate(nodes);
 
     evaluationContext = backupContext;
     return Value(nodes, Value::adopt);
 }
 
@@ skipping 86 matching lines @@
     Value v = m_filter->evaluate();
 
     NodeSet& nodes = v.modifiableNodeSet();
     m_path->evaluate(nodes);
 
     return v;
 }
 
 }
 }