Externs for Element.prototype.getAttributeNames.

This was first attempted in [] but it broke several targets that depended on an existing definition and had to be rolled back.

Global presubmit (Including all the tests that failed previously): []

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Created by MOE: https://github.com/google/moe
MOE_MIGRATED_REVID=165213223

externs/browser/w3c_dom4.js

@@ -67,3 +67,9 @@ Element.prototype.replaceWith = function(nodes) {};
  * @see https://dom.spec.whatwg.org/#dom-childnode-replacewith
  */
 CharacterData.prototype.replaceWith = function(nodes) {};
+
+/**
+ * @return {!Array<string>}
+ * @see https://dom.spec.whatwg.org/#dom-element-getattributenames
+ */
+Element.prototype.getAttributeNames = function() {};