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

@jakearchibald, I'm kind of confused by https://github.com/w3c/ServiceWorker/issues/1091#issuecomment-291048685.  I don't recall this being the outcome from the f2f meeting.

I thought we said we were going to do:

* client: the client initiating the load just like we do for non-replacement cases
* reserved client: the client currently attached to the about:blank window
* target client: I don't remember, but the about:blank window would make sense to me.

If you don't want the existing about:blank to be "reserved" since its already execution ready, we could also do:

* client: the client initiating the load just like we do for non-replacement cases
* reserved client: null
* target client: I don't remember, but the about:blank window would make sense to me.

So in:

```
// foo.html
let f = document.createElement('iframe');
document.body.appendChild(f);

f.contentWindow.someGlobalStatus = 'yuck';

f.src = 'frame.html';
```

I guess I would always expect `FetchEvent.clientId` to reference the foo.html parent window here.

It also seems to make sense to make `FetchEvent.targetClientId` reference the current about:blank iframe client.

I can go either way about whether `FetchEvent.reservedClientId` should be null or the iframe client.  I guess from an implementation point of view null would be slightly more convenient for me.

@smaug---- also was wondering what we do if replacement load is for a download.  The example he gave was:

```
win = window.open(); win.document.body.innerHTML = "<a href='http://download.me.com/foo.zip'>zip</a>"
```

In this case, if someone clicked the link I would expect:

* the `.clientId` to be the about:blank window (since its an anchor tag),
* the reserved client to be null (if we decide to go that way above)
* the target client to be the about:blank window since we don't know yet if it will download

If it was `<a download>` then I would expect the target client to be null.

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