Add metric to record the BrowserSignin policy value.

A metric for all BrowserSignins exists, but an individual metric for each policy value was not accounted for. Specific metrics for each policy value were added and can be viewed in the Enterprise.BrowserSigninPolicy histogram. PolicyStatisticsCollector was used to run the metric recording since BrowserSigninPolicyHandler is applied multiple times at startup.

Bug: 1248141
Change-Id: I87901c3bf0b5420693ca3071d102a75c36a8e076
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/3836772


Reviewed-by: Owen Min <zmin@chromium.org>
Commit-Queue: Alex Chen <alexwchen@chromium.org>
Cr-Commit-Position: refs/heads/main@{#1038344}