[w3c/ServiceWorker] Can respondWith() be called from a microtask? (#1394)

Should this respondWith() throw?
```
self.addEventListener('fetch', e => {
  Promise.resolve().then(() => {
    e.respondWith(new Response('respondWith succeeded'));
  });
```

Firefox and Chrome have different behavior here. Demo site at
https://respond-with-microtask.glitch.me/
(Chrome Canary throws an exception and then crashes, sorry, just ignore the crash for now.)

In Chrome we tried hard to make it so respondWith()/waitUntil() can't be called like this, now I'm wondering whether the spec allows it after all.

The respondWith() spec steps check whether the dispatch flag is set: https://dom.spec.whatwg.org/#dispatch-flag 

It's not clear to me if the dispatch flag gets unset before microtasks are run at the end of the event handler.

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