[ServiceWorker] should SW MessageEvents inherit ExtendableEvent? (#690)

The spec has been changed so that most events fired on ServiceWorker inherit from ExtendableEvent and therefore have a `.waitUntil()` method.  The one exception is MessageEvent.  Should we have some kind of ExtendedMessageEvent that inherits from ExtendableEvent and MessageEvent?  Right now one cannot do this:

```
addEventListener('message', function(event) {
  event.waitUntil(cache.add(event.data.request));
});
```

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Reply to this email directly or view it on GitHub:
https://github.com/slightlyoff/ServiceWorker/issues/690