Introduce new policy source: restricted override

The restricted policy source is used to indicate the hardcoded policy
overrides by the restricted MGS policy. This source has a higher
priority than the values set by the admin via server UI without causing
any conflicts.

This way, the source is also clear for the admin when inspecting the
chrome://policy page for debugging purposes.

Bug: b:186737181
Change-Id: If9d25e66bf961d5c06fa26b3753d4b9101c5cf5e
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2978572
Commit-Queue: Maria Petrisor <mpetrisor@chromium.org>
Reviewed-by: Roland Bock <rbock@google.com>
Reviewed-by: Pavol Marko <pmarko@chromium.org>
Reviewed-by: Alexander Hendrich <hendrich@chromium.org>
Cr-Commit-Position: refs/heads/master@{#900372}