Implement formdata event

components/script/dom/webidls/FormDataEvent.webidl

@@ -0,0 +1,14 @@
+/* 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 https://mozilla.org/MPL/2.0/. */
+
+// https://html.spec.whatwg.org/multipage/#the-formdataevent-interface
+[Exposed=Window,
+ Constructor(DOMString type, optional FormDataEventInit eventInitDict)]
+interface FormDataEvent : Event {
+  readonly attribute FormData formData;
+};
+
+dictionary FormDataEventInit : EventInit {
+  /*required*/ FormData formData;
+};