Update DOMTokenList::value() to return a const reference

Source/core/dom/DOMTokenList.h

 /*
  * Copyright (C) 2010 Google 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.
  * 2.  Redistributions in binary form must reproduce the above copyright
  *     notice, this list of conditions and the following disclaimer in the
@@ skipping 39 matching lines @@
     virtual const AtomicString item(unsigned index) const = 0;
 
     bool contains(const AtomicString&, ExceptionState&) const;
     virtual void add(const Vector<String>&, ExceptionState&);
     void add(const AtomicString&, ExceptionState&);
     virtual void remove(const Vector<String>&, ExceptionState&);
     void remove(const AtomicString&, ExceptionState&);
     bool toggle(const AtomicString&, ExceptionState&);
     bool toggle(const AtomicString&, bool force, ExceptionState&);
 
-    AtomicString toString() const { return value(); }
+    const AtomicString& toString() const { return value(); }
 
     virtual Element* element() { return 0; }
 
 protected:
-    virtual AtomicString value() const = 0;
+    virtual const AtomicString& value() const = 0;
     virtual void setValue(const AtomicString&) = 0;
 
     virtual void addInternal(const AtomicString&);
     virtual bool containsInternal(const AtomicString&) const = 0;
     virtual void removeInternal(const AtomicString&);
 
     static bool validateToken(const String&, ExceptionState&);
     static bool validateTokens(const Vector<String>&, ExceptionState&);
     static AtomicString addToken(const AtomicString&, const AtomicString&);
     static AtomicString addTokens(const AtomicString&, const Vector<String>&);
     static AtomicString removeToken(const AtomicString&, const AtomicString&);
     static AtomicString removeTokens(const AtomicString&, const Vector<String>&);
 };
 
 } // namespace WebCore
 
 #endif // DOMTokenList_h