Bug 1270364. MimeTypeArray should have unenumerable named properties per spec. r=bkelly

dom/webidl/MimeTypeArray.webidl

@@ -4,9 +4,7 @@
  * You can obtain one at http://mozilla.org/MPL/2.0/.
  */
 
-// [LegacyUnenumerableNamedProperties]
-// Named properties enumerable for now; see
-// https://bugzilla.mozilla.org/show_bug.cgi?id=1270364
+[LegacyUnenumerableNamedProperties]
 interface MimeTypeArray {
   readonly attribute unsigned long length;