ServiceWorker: Introduce ExtendableMessageEvent to replace MessageEvent

Source/modules/serviceworkers/ExtendableMessageEventInit.idl

Index: Source/modules/serviceworkers/ExtendableMessageEventInit.idl
diff --git a/Source/modules/serviceworkers/ServiceWorkerMessageEventInit.idl b/Source/modules/serviceworkers/ExtendableMessageEventInit.idl
similarity index 68%
copy from Source/modules/serviceworkers/ServiceWorkerMessageEventInit.idl
copy to Source/modules/serviceworkers/ExtendableMessageEventInit.idl
index 9c134ece7c14b44233fe7c3278cfd94a9b4ec5c7..70155257038cd7d460b856beca64bb115b3c63b4 100644
--- a/Source/modules/serviceworkers/ServiceWorkerMessageEventInit.idl
+++ b/Source/modules/serviceworkers/ExtendableMessageEventInit.idl
@@ -2,12 +2,12 @@
 // Use of this source code is governed by a BSD-style license that can be
 // found in the LICENSE file.
 
-// https://slightlyoff.github.io/ServiceWorker/spec/service_worker/#serviceworkermessage-event-init-dictionary
+// https://slightlyoff.github.io/ServiceWorker/spec/service_worker/#extendablemessage-event-interface
 
-dictionary ServiceWorkerMessageEventInit : EventInit {
+dictionary ExtendableMessageEventInit: ExtendableEventInit {
     any data;
     DOMString origin;
     DOMString lastEventId;
-    (ServiceWorker or MessagePort)? source;
+    (Client or ServiceWorker or MessagePort)? source;
     sequence<MessagePort> ports;
 };