Sync the DOMSettableTokenList interface with the spec

Source/core/dom/DOMSettableTokenList.idl

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+// https://dom.spec.whatwg.org/#interface-domsettabletokenlist
+
 interface DOMSettableTokenList : DOMTokenList {
+    attribute DOMString value;
+
+    // FIXME: This getter is not in the spec.
     [ImplementedAs=item] getter DOMString? (unsigned long index);
-    attribute DOMString value;
 };
-