The expected behavior is that if the SW does something like:

  e.waitUntil(p);
  e.respondWith(r);

Then the browser starts processing `r` but keeps the SW alive until `p` settles. In C++ this is handled by:

  r settles => Mojo OnResponse* is called => SWFD runs fetch_callback_.
  p settles => Mojo response to DispatchFetchEvent is called => SWFD calls version_->FinishRequest().

But it looks like SWFD is calling FinishRequest)() once an OnResponse* is called.

So it seems to me SWFD isn't waiting for waitUntil.