Re-land: bindings: Use MessageEventInit for MessageEvent constructor

Source/core/events/MessageEventInit.idl

Index: Source/core/events/MessageEventInit.idl
diff --git a/Source/core/events/MessageEventInit.idl b/Source/core/events/MessageEventInit.idl
new file mode 100644
index 0000000000000000000000000000000000000000..d25296560e227796bf7456995fcd93ad5e6f11ed
--- /dev/null
+++ b/Source/core/events/MessageEventInit.idl
@@ -0,0 +1,16 @@
+// 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://html.spec.whatwg.org/multipage/comms.html#the-messageevent-interfaces
+
+dictionary MessageEventInit : EventInit {
+    any data;
+    DOMString origin;
+    DOMString lastEventId;
+    // TODO(bashi): |source| should be (WindowProxy or MessagePort)?
+    EventTarget source;
+    // Per spec, |ports| isn't nullable, but it seems it should be.
+    // https://www.w3.org/Bugs/Public/show_bug.cgi?id=23176
+    sequence<MessagePort>? ports;
+};