Make HTMLOutputElement.htmlFor settable

LayoutTests/fast/dom/HTMLOutputElement/script-tests/dom-settable-token-list.js

 // This test mostly comes from fast/dom/HTMLElement/script-tests/class-list.js
 description('Tests the htmlFor attribute and its properties.');
 
 var container = document.createElement('div');
 document.body.appendChild(container);
 
 var element;
 
 function createElement(tokenList)
 {
     container.innerHTML = '<output for="' + tokenList + '"></output>';
     element = container.lastChild;
 }
 
 debug('- Tests from http://simon.html5.org/test/html/dom/reflecting/DOMTokenList/');
 
-// HTMLOutputElement::htmlFor is readonly attribute.
+// HTMLOutputElement::htmlFor setter is forwarding assignment to DOMSettableTokenList.value attribute.
 createElement('x');
+shouldBeEqualToString('element.htmlFor.value', 'x');
+shouldBeEqualToString('String(element.htmlFor)', 'x');
 element.htmlFor = 'y';
-shouldBeEqualToString('String(element.htmlFor)', 'x');
+shouldBeEqualToString('element.htmlFor.value', 'y');
+shouldBeEqualToString('String(element.htmlFor)', 'y');
 
 // http://simon.html5.org/test/html/dom/reflecting/DOMTokenList/getting/001.htm
 createElement('');
 shouldEvaluateTo('element.htmlFor.length', 0);
 
 // http://simon.html5.org/test/html/dom/reflecting/DOMTokenList/getting/002.htm
 createElement('x');
 shouldEvaluateTo('element.htmlFor.length', 1);
 
 // http://simon.html5.org/test/html/dom/reflecting/DOMTokenList/getting/003.htm
@@ skipping 212 matching lines @@
 shouldBeTrue('\'undefined\' != typeof DOMSettableTokenList');
 
 shouldBeEqualToString('typeof DOMSettableTokenList.prototype', 'object');
 
 createElement('x');
 shouldBeEqualToString('typeof element.htmlFor', 'object');
 
 shouldEvaluateTo('element.htmlFor.constructor', 'DOMSettableTokenList');
 
 shouldBeTrue('element.htmlFor === element.htmlFor');