[ServiceWorker] Consider dropping initServiceWorkerMessageEvent() (#701)

`ServiceWorkerMessageEvent` has `initServiceWorkerMessageEvent()` [1], but I think we should drop this method because as the note[2] says, `initEvent()` method only exists for historical reason and we can use constructor to initialize events.

[1] https://slightlyoff.github.io/ServiceWorker/spec/service_worker/#serviceworkermessage-event-initserviceworkermessageevent-method
[2] https://dom.spec.whatwg.org/#dom-event-initeventtype-bubbles-cancelable

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Reply to this email directly or view it on GitHub:
https://github.com/slightlyoff/ServiceWorker/issues/701