GitLab

Adds code so KerberosCredentialsManager can listen to the 4 Kerberos
prefs and handles kerberos.enabled (removes all existing accounts).

Moves the registration of 3 prefs from profile to local_state since
those prefs were not per-profile, anyway, and it simplifies lifetime of
KerberosCredentialsManager as it does not have to wait for the primary
user to be available.

Also changes lifetime from a static singleton to explicit management
since KerberosCredentialsManager owns a pointer to local_state now and
it would otherwise be hard to justify that the pointer is never
dangling.

TBR=battre@chromium.org

BUG=chromium:952239
TEST=Turn KerberosEnabled policy on, navigate to
     chrome://settings/kerberosAccounts, add an account,
     Turn KerberosEnabled policy off, verify that the account vanishes.

Change-Id: I3f4ed443014726d8b2c5f2cc536703badf37d2ae
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1617455
Commit-Queue: Lutz Justen <ljusten@chromium.org>
Reviewed-by: Dominic Battré <battre@chromium.org>
Reviewed-by: Xiyuan Xia <xiyuan@chromium.org>
Reviewed-by: Pavol Marko <pmarko@chromium.org>
Cr-Commit-Position: refs/heads/master@{#664432}