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

Source/core/dom/DOMTokenList.h

Index: Source/core/dom/DOMTokenList.h
diff --git a/Source/core/dom/DOMTokenList.h b/Source/core/dom/DOMTokenList.h
index bdaf34de64418a2ed14dd49cf2883851a2eff5f5..bd5204e9f12f726caace85316090e240137158de 100644
--- a/Source/core/dom/DOMTokenList.h
+++ b/Source/core/dom/DOMTokenList.h
@@ -57,12 +57,12 @@ public:
     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&);