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

Source/core/html/ClassList.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 44 matching lines @@
 
     void clearValueForQuirksMode() { m_classNamesForQuirksMode = nullptr; }
 
 private:
     ClassList(Element*);
 
     virtual bool containsInternal(const AtomicString&) const OVERRIDE;
 
     const SpaceSplitString& classNames() const;
 
-    virtual AtomicString value() const OVERRIDE { return m_element->getAttribute(HTMLNames::classAttr); }
+    virtual const AtomicString& value() const OVERRIDE { return m_element->getAttribute(HTMLNames::classAttr); }
     virtual void setValue(const AtomicString& value) OVERRIDE { m_element->setAttribute(HTMLNames::classAttr, value); }
 
     Element* m_element;
     mutable OwnPtr<SpaceSplitString> m_classNamesForQuirksMode;
 };
 
 } // namespace WebCore
 
 #endif // ClassList_h