Implement onunhandledrejection / onrejectionhandled events

Source/core/events/PromiseRejectionEventInit.idl

Index: Source/core/events/PromiseRejectionEventInit.idl
diff --git a/Source/core/events/PromiseRejectionEventInit.idl b/Source/core/events/PromiseRejectionEventInit.idl
new file mode 100644
index 0000000000000000000000000000000000000000..cf6b3bae5131bcb65afd69c4cb668cfd4be7e8fc
--- /dev/null
+++ b/Source/core/events/PromiseRejectionEventInit.idl
@@ -0,0 +1,12 @@
+// Copyright 2015 The Chromium Authors. All rights reserved.
+// Use of this source code is governed by a BSD-style license that can be
+// found in the LICENSE file.
+
+// https://github.com/domenic/unhandled-rejections-browser-spec
+
+[
+    RuntimeEnabled=PromiseRejectionEvent,
+] dictionary PromiseRejectionEventInit : EventInit {
+    Promise<any> promise;
+    any reason;
+};