Minor simplifications for use_count and use_date in EntityTable

Follow-ups to comments on http://crrev.com/c/6477428. This changes
the interpretation of the values in the use_date column, but the CL
should be safe to land as a fast follow-up.

Bug: 408421018
Change-Id: I68c8839c7f2399fa03e66af60f1b2f8817a5b397
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/6491992
Commit-Queue: Christoph Schwering <schwering@google.com>
Commit-Queue: Jan Keitel <jkeitel@google.com>
Reviewed-by: Christoph Schwering <schwering@google.com>
Auto-Submit: Jan Keitel <jkeitel@google.com>
Cr-Commit-Position: refs/heads/main@{#1452200}