ServiceWorker controller mismatch when prefetch is served

[hiroshige-g]

This is a kind of meta issue to track the mismatch between:

1. Controlling ServiceWorker at the time of prefetch, which is set as prefetch record's navigation param's reserved client's active service worker.
2. Controlling ServiceWorker of the navigated Document when served by prefetch (if no fixes are made).
3. Controlling ServiceWorker of the navigated Document that would be used without prefetch.
4. Controlling ServiceWorker of the navigated Document after claim() when served by prefetch.

Naively I'd like to expect [2][3][4] should be the same but they can be different for several reasons:

• After prefetch before navigation, a newer ServiceWorker version is added in the ServiceWorker registration used for [1]: [1]/[2] is the old version, [3]/[4] is the new version. If we evict prefetch records on newer version (Prefetch requests keeping the service worker alive and preventing it from revving its version #347), then this is no longer a problem.
• After prefetch before navigation, a new ServiceWorker Registration covering the prefetched URL is added: [1]/[2] is the old registration, [3]/[4] is the new registration.
• No-Vary-Search (No-Vary-Search can cause reserved client creation URL mismatch #350): [1][2][4] is the controller based on prefetch URL, [3] is the controller based on navigation URL.

I and @nhiroki discussed an option to compute the service worker controller at the time of navigation/prefetch record matching (==[3]) and compare it against [1], to avoid this kind of mismatch.
However, I found this might be harder than expected. This is probably doable (not sure in the short-term though) and I'll try another iteration later.