Update WebXR Layer IDL, and matching changes to the Rust code.

components/script/dom/webidls/XRLayerEvent.webidl

@@ -0,0 +1,15 @@
+/* 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://immersive-web.github.io/layers/#xrlayerevent-interface
+
+// [SecureContext, Exposed=Window]
+// interface XRLayerEvent : Event {
+//   constructor(DOMString type, XRLayerEventInit eventInitDict);
+//   [SameObject] readonly attribute XRLayer layer;
+// };
+
+dictionary XRLayerEventInit : EventInit {
+  required XRLayer layer;
+};