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

Source/core/dom/DOMSettableTokenList.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 34 matching lines @@
 
     virtual void ref() OVERRIDE { RefCounted<DOMSettableTokenList>::ref(); }
     virtual void deref() OVERRIDE { RefCounted<DOMSettableTokenList>::deref(); }
 
     virtual unsigned length() const OVERRIDE { return m_tokens.size(); }
     virtual const AtomicString item(unsigned index) const OVERRIDE;
 
     virtual void add(const Vector<String>&, ExceptionState&) OVERRIDE;
     virtual void remove(const Vector<String>&, ExceptionState&) OVERRIDE;
 
-    virtual AtomicString value() const OVERRIDE { return m_value; }
+    virtual const AtomicString& value() const OVERRIDE { return m_value; }
     virtual void setValue(const AtomicString&) OVERRIDE;
 
     const SpaceSplitString& tokens() const { return m_tokens; }
 
 protected:
     DOMSettableTokenList();
 
 private:
     virtual void addInternal(const AtomicString&) OVERRIDE;
     virtual bool containsInternal(const AtomicString&) const OVERRIDE;
     virtual void removeInternal(const AtomicString&) OVERRIDE;
 
     AtomicString m_value;
     SpaceSplitString m_tokens;
 };
 
 } // namespace WebCore
 
 #endif // DOMSettableTokenList_h