Add UMA for website familiarity

This CL adds the SiteFamiliarityHeuristicResult UMA. The UMA records
different heuristics for determining the user's familiarity with a
website. The UMA will be used to evaluate different heuristics for
enabling site protections on different sites similar to what the Edge
browser does for "Enhanced Site Security".

BUG=361215225
TEST=SiteProtectionMetricsObserverTest.*

Change-Id: I4d549815d7d1b16d10870436b11c0ddf201c28b2
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/5792517


Reviewed-by: Xinghui Lu <xinghuilu@chromium.org>
Commit-Queue: Peter Kotwicz <pkotwicz@chromium.org>
Reviewed-by: Yaron Friedman <yfriedman@chromium.org>
Reviewed-by: Ella Ge <eirage@chromium.org>
Reviewed-by: Joe DeBlasio <jdeblasio@chromium.org>
Cr-Commit-Position: refs/heads/main@{#1349447}