SiteIsolation: Erase destroyed host after logging reuse metric

The `TimeSinceReusableProcessDestroyed` metric was being inflated
because a destroyed host's entry was not removed after being recorded.
This allowed a single destroyed host to be counted multiple times on
subsequent lookups.

This change erases the entry after use to ensure it is only logged once.
The histograms are renamed to isolate data from the new logic.

Bug: 403216940

Change-Id: I9016d9c3c5a6b146a79d22cb4dd4a5b90f8a71c4
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/6956388
Reviewed-by: Camille Lamy <clamy@chromium.org>
Commit-Queue: Alex Attar <aattar@google.com>
Cr-Commit-Position: refs/heads/main@{#1520696}