Tidy finds WebIDLs with no spec.

Closes #6689

components/script/dom/webidls/ErrorEvent.webidl

@@ -2,6 +2,8 @@
  * License, v. 2.0. If a copy of the MPL was not distributed with this
  * file, You can obtain one at http://mozilla.org/MPL/2.0/. */
 
+// https://html.spec.whatwg.org/multipage/webappapis.html#the-errorevent-interface 
+
 [Constructor(DOMString type, optional ErrorEventInit eventInitDict)/*, Exposed=(Window,Worker)*/]
 interface ErrorEvent : Event {
   readonly attribute DOMString message;