Support native promise callbacks.

components/script/dom/webidls/PromiseNativeHandler.webidl

@@ -0,0 +1,13 @@
+/* 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/. */
+
+// This interface is entirely internal to Servo, and should not be accessible to
+// web pages.
+
+// Hack to allow us to have JS owning and properly tracing/CCing/etc a
+// PromiseNativeHandler.
+[NoInterfaceObject,
+ Exposed=(Window,Worker)]
+interface PromiseNativeHandler {
+};