Validate HashPrefixMapView is not being changed during VerifyChecksum

We see some new crashes in VerifyChecksum starting in M131. Most of the
mmap cleanup landed in M130, so this is surprising.
https://crrev.com/c/5955393, however was landed in M132 and
cherry-picked back to M131. As an easy first step toward debugging,
DCHECK that the map truly is the same size. This could flush out
concurrent modifications of the DB.

This CL also removes an unused method that accesses the HashPrefixMap,
removing one place we have to check for modification.

Bug: 390144275, 41356806
Change-Id: I0fa000097d0fba0273de062e35a32c3bff5be790
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/6370311
Reviewed-by: thefrog <thefrog@chromium.org>
Commit-Queue: Daniel Rubery <drubery@chromium.org>
Cr-Commit-Position: refs/heads/main@{#1435086}