Add crash keys to V4Store::VerifyChecksum

To confirm if shutdown hangs are due to some kind of corruption leading
to making checksum verification take extraordinarily long, this CL adds
crash keys that will give us a sense of how long checksum computation
should take.

Bug: 331054795
Change-Id: Ia0ac7fb77ac3cb31d501284705aae1c11a2059de
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/5398267


Commit-Queue: Daniel Rubery <drubery@chromium.org>
Reviewed-by: thefrog <thefrog@chromium.org>
Cr-Commit-Position: refs/heads/main@{#1278978}