ServiceWorker: Introduce ExtendableMessageEvent to replace MessageEvent

Source/modules/serviceworkers/ExtendableMessageEvent.idl

Index: Source/modules/serviceworkers/ExtendableMessageEvent.idl
diff --git a/Source/modules/serviceworkers/ServiceWorkerMessageEvent.idl b/Source/modules/serviceworkers/ExtendableMessageEvent.idl
similarity index 61%
copy from Source/modules/serviceworkers/ServiceWorkerMessageEvent.idl
copy to Source/modules/serviceworkers/ExtendableMessageEvent.idl
index 854b9a88dafca7603c1228c2816c9e61787b4a2f..e7686eaf5e31ba911c7d3c735cea7bd0e10f5bd1 100644
--- a/Source/modules/serviceworkers/ServiceWorkerMessageEvent.idl
+++ b/Source/modules/serviceworkers/ExtendableMessageEvent.idl
@@ -2,16 +2,16 @@
 // 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-interface
+// https://slightlyoff.github.io/ServiceWorker/spec/service_worker/#extendablemessage-event-interface
 
 [
-    Constructor(DOMString type, optional ServiceWorkerMessageEventInit eventInitDict),
-    Exposed=(Window, Worker),
+    Constructor(DOMString type, optional ExtendableMessageEventInit eventInitDict),
+    Exposed=ServiceWorker,
     RuntimeEnabled=ServiceWorker,
-] interface ServiceWorkerMessageEvent : Event {
-    [Custom=Getter] readonly attribute any data;
+] interface ExtendableMessageEvent : ExtendableEvent {
     readonly attribute DOMString origin;
     readonly attribute DOMString lastEventId;
-    readonly attribute (ServiceWorker or MessagePort)? source;
+    readonly attribute (Client or ServiceWorker or MessagePort)? source;
+    [Custom=Getter] readonly attribute any data;
     readonly attribute MessagePort[]? ports;
 };