Re: [w3c/ServiceWorker] consider Client behavior for windows where initial about:blank is replaced with a loaded document (#1091)

I'll post the test once I have it written.  Its revealing more problems for me that I'm trying to work through.

One thing that I have noticed is that service workers may have exposed some browser optimizations that were not observable before.

Consider loading a simply `<iframe src="foo.html"></iframe>`.  Per the spec this should create an initial "about:blank".  At least in gecko, though, we don't actually do this since nothing tries to access `.contentWindow`.  This allows us to save some work when nothing has interacted with the frame before the load completes.

With service workers, though, we can do this:

1. Intercept foo.html and delay the response
2. Do a clients.matchAll() to observe the initial about:blank

In this case our optimization to avoid creating the initial about:blank is exposed because step (2) won't find a Client.

At least with our current Clients implementation anyway.  I'm trying to make this work without forcing a de-opt in the general case.  I'm trying to make our code create the initial about:blank on demand when `clients.matchAll()` would have observed it.

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