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

Source/core/dom/DOMSettableTokenList.h

Index: Source/core/dom/DOMSettableTokenList.h
diff --git a/Source/core/dom/DOMSettableTokenList.h b/Source/core/dom/DOMSettableTokenList.h
index dd5b93a2932132fd5b721e84a2947261645cc48e..2a59e85b8197b562e941aa7bc0716fbef7534ead 100644
--- a/Source/core/dom/DOMSettableTokenList.h
+++ b/Source/core/dom/DOMSettableTokenList.h
@@ -52,7 +52,7 @@ public:
     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; }