[memories] Cap the related searches count for top visits.

This applies a cap of 8 to the related searches count of the top visit.
We can still exceed the cap, since we grab a whole visits' worth of
searches at a time. But we won't grab any more after exceeding it.

In practice we expect all visits to have either 0 or 8 related searches,
but this code should be robust against visits that strangely have fewer
or more related searches too.

Bug: 1245781, 1171352
Change-Id: Ic26ade4a4e404e2b3ffc5c30abd82fb0fb8fef95
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/3166919
Reviewed-by: Moe Ahmadi <mahmadi@chromium.org>
Commit-Queue: Tommy Li <tommycli@chromium.org>
Cr-Commit-Position: refs/heads/main@{#922567}