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Wire NetworkIsolationKey through DomainReliabilityContext.

This is the interesting CL of the bunch. To keep things simple, it keeps
all pending beacons in a single list with a maximum size, even with
different NetworkIsolationKeys. To upload a report, it selects all
beacon with the same Key as the first (oldest) beacon and creates a
report using them. On successful upload, it will remove just those
beacons, and try to upload again (with backoff), if any beacons are
left.

This CL also adds some eviction tests, and removes some incorrect
eviction DCHECKs, which assumed that all beacons currently being
uploaded could not be evicted. That may be unlikely, but there's
nothing from preventing it from actually happening, and it may
be more likely once only beacons with a matching NIK are uploaded
each time.

Bug: 1138994
Change-Id: I37d97b679cc0309c5e128c239585555ab097369b
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2587669
Commit-Queue: Matt Menke <mmenke@chromium.org>
Reviewed-by: Lily Chen <chlily@chromium.org>
Cr-Commit-Position: refs/heads/master@{#837146}