Add a dummy implementation of all the XR Layer types

components/script/dom/webidls/XRCubeLayer.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 https://mozilla.org/MPL/2.0/. */
+
+// TODO: Implement the layer types
+// https://github.com/servo/servo/issues/27493
+
+// https://immersive-web.github.io/layers/#xrcubelayer
+
+[SecureContext, Exposed=Window, Pref="dom.webxr.layers.enabled"]
+interface XRCubeLayer : XRCompositionLayer {
+//   attribute XRSpace space;
+//   attribute DOMPointReadOnly orientation;
+//
+//   // Events
+//   attribute EventHandler onredraw;
+};