ServiceWorkerMessageEvent shouldn't hold ScriptValue

third_party/WebKit/Source/modules/serviceworkers/ServiceWorkerMessageEvent.idl

 // 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://slightlyoff.github.io/ServiceWorker/spec/service_worker/#serviceworkermessage-event-interface
 
 [
-    Constructor(DOMString type, optional ServiceWorkerMessageEventInit eventInitDict),
+    // TODO(bashi): Stop using CustomConstructor once we solve reference
+    // circulation between Blink and V8. http://crbug.com/501866
+    // Constructor should be:
+    // Constructor(DOMString type, optional ServiceWorkerMessageEventInit eventInitDict),
+    CustomConstructor,
     Exposed=(Window, Worker),
 ] interface ServiceWorkerMessageEvent : Event {
     [Custom=Getter] readonly attribute any data;
     readonly attribute DOMString origin;
     readonly attribute DOMString lastEventId;
     readonly attribute (ServiceWorker or MessagePort)? source;
     readonly attribute MessagePort[]? ports;
 };