[History] Use fixed time of day to deflake VisitDatabase unittest

VisitDatabaseTest.GetDailyVisitsToOrigin_404s was disabled for flaking,
specifically on the number of days with visits. The test inserts two
visits separated by an hour and expects them to have occurred on the
same day. The test was originally written using base::Time::Now() as the
time of the first visit, meaning the one-hour difference to the next
visit could cross into the following day if the test was run close to
midnight, creating two days with visits rather than one. Changing the
base time to a fixed time of day more than hour before the end of the
day should prevent this flaky behavior.

Bug: 448019671
Change-Id: I430fc659fc46dd342d0a91fed5887ff1606f255c
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/6995360
Commit-Queue: Svend Larsen <svend@chromium.org>
Reviewed-by: manuk hovanesian <manukh@chromium.org>
Cr-Commit-Position: refs/heads/main@{#1522834}