Use ExtendableMessageEvent for messageerror in service workers. (#1485)

It seems inconsistent to use ExtendableMessageEvent for message events but not for messageerror events. This fixes that inconsistency.

Author: mkruisselbrink

docs/index.bs

@@ -456,10 +456,11 @@ spec: webappsec-referrer-policy; urlPrefix: https://w3c.github.io/webappsec-refe
                 1. Let |destination| be the {{ServiceWorkerGlobalScope}} object associated with |serviceWorker|.
                 1.  Let |deserializeRecord| be <a abstract-op>StructuredDeserializeWithTransfer</a>(|serializeWithTransferResult|, |destination|'s [=global object/Realm=]).
 
-                    If this throws an exception, catch it, [=fire an event=] named {{messageerror!!event}} at |destination|, using {{MessageEvent}}, with the {{MessageEvent/origin}} attribute initialized to |origin| and the {{MessageEvent/source}} attribute initialized to |source|, and then abort these steps.
-                1. Let |messageClone| be |deserializeRecord|.\[[Deserialized]].
-                1. Let |newPorts| be a new [=frozen array type|frozen array=] consisting of all {{MessagePort}} objects in |deserializeRecord|.\[[TransferredValues]], if any, maintaining their relative order.
-                1. Let |e| be the result of [=creating an event=] named {{message!!event}}, using {{ExtendableMessageEvent}}, with the {{ExtendableMessageEvent/origin}} attribute initialized to |origin|, the {{ExtendableMessageEvent/source}} attribute initialized to |source|, the {{ExtendableMessageEvent/data}} attribute initialized to |messageClone|, and the {{ExtendableMessageEvent/ports}} attribute initialized to |newPorts|.
+                    If this throws an exception, let |e| be the result of [=creating an event=] named {{messageerror!!event}}, using {{ExtendableMessageEvent}}, with the {{ExtendableMessageEvent/origin}} attribute initialized to |origin| and the {{ExtendableMessageEvent/source}} attribute initialized to |source|.
+                1. Else:
+                    1. Let |messageClone| be |deserializeRecord|.\[[Deserialized]].
+                    1. Let |newPorts| be a new [=frozen array type|frozen array=] consisting of all {{MessagePort}} objects in |deserializeRecord|.\[[TransferredValues]], if any, maintaining their relative order.
+                    1. Let |e| be the result of [=creating an event=] named {{message!!event}}, using {{ExtendableMessageEvent}}, with the {{ExtendableMessageEvent/origin}} attribute initialized to |origin|, the {{ExtendableMessageEvent/source}} attribute initialized to |source|, the {{ExtendableMessageEvent/data}} attribute initialized to |messageClone|, and the {{ExtendableMessageEvent/ports}} attribute initialized to |newPorts|.
                 1. [=Dispatch=] |e| at |destination|.
                 1. Invoke [=Update Service Worker Extended Events Set=] with |serviceWorker| and |e|.
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