[whatwg/xhr] Expose ProgressEvent to ServiceWorker (#305)

FileReader is exposed to ServiceWorker and uses ProgressEvent, but the event interface is not exposed to ServiceWorker since 9d0a9bb85c32e10788227a79943362bede178a7c.

Exposing it to ServiceWorker also matches Gecko and WebKit implementation:

* Gecko https://searchfox.org/mozilla-central/rev/2aea8e093c3c6d10bc3f385e0ea66b6387221e71/dom/webidl/ProgressEvent.webidl
* WebKit https://webkit-search.igalia.com/webkit/rev/7418d7c94bd380d0844789d128e5805e28d9c572/Source/WebCore/dom/ProgressEvent.idl

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