Re: [ServiceWorker] Specify .focus() and .openWindow() behaviour (#602)

Chrome's current implementation doesn't allow `focus()` inside of a `fetch` handler. I'd like to make sure we allow it for top-level navigations, but would understand if we want to preclude it for non-top-level navigations. My read of the "allowed to show a popup" clause [1] doesn't make it clear what we should (dis)allow as that language is very much about a single document's script context, not about multi-document interaction.

Thoughts? 

[1]: https://html.spec.whatwg.org/multipage/browsers.html#allowed-to-show-a-popup

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