[ServiceWorker] FetchEvent IDL issues (#680)

`request` and `client` are initialized to null but are not nullable. It's not clear to me that `request` should be nullable, perhaps we should just require it to be always provided now that is possible?

Furthermore, the dictionary should offer default values (except if the member is required).

---
Reply to this email directly or view it on GitHub:
https://github.com/slightlyoff/ServiceWorker/issues/680