implement ExtendableEvent as base type for ServiceWorker events

components/script/dom/webidls/ExtendableEvent.webidl

@@ -0,0 +1,17 @@
+/* This Source Code Form is subject to the terms of the Mozilla Public
+ * 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://w3c.github.io/ServiceWorker/#extendable-event
+
+[Constructor(DOMString type,
+ optional ExtendableEventInit eventInitDict),
+ Exposed=ServiceWorker,
+ Pref="dom.serviceworker.enabled"]
+interface ExtendableEvent : Event {
+  [Throws] void waitUntil(/*Promise<*/any/*>*/ f);
+};
+
+dictionary ExtendableEventInit : EventInit {
+  // Defined for the forward compatibility across the derived events
+};