Re: [w3c/ServiceWorker] ServiceWorkerMessageEvent isn't consistent with MessageEvent (#989)

Having looked at the [previous discussion around this](https://github.com/w3c/ServiceWorker/issues/669), we wanted the same for `ServiceWorkerMessageEvent` at the beginning. Later, for `ExtendableMessageEvent`, we had a reason not having been able to extend `MessageEvent`. I think even if we merge `ServiceWorkerMessageEvent` and `MessageEvent` at this time, we anyway have to maintain `ExtendableMessageEvent` interface separately as it's not a `MessageEvent`. Considering they have been shipped for a while, I'm inclined to regard them as pair of events defined in SW spec.

/cc @jakearchibald 

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