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In addition to the kUserCrostiniAllowedByPolicy preference, there are
two other ways in which Crostini can be disallowed by policy:
unaffiliated user restrictions, and by disallowing virtual machines.
This CL changes the behavior of the Linux row in Chrome Settings so that
it shows a policy indicator icon & disables the "turn on" button when
Crostini is disallowed by one of these factors.

Change-Id: Iac6f3ca4c98be74308cb4177eadfb34c1d92d635
Reviewed-on: https://chromium-review.googlesource.com/c/1325594
Commit-Queue: Renée Wright <rjwright@chromium.org>
Reviewed-by: Nicholas Verne <nverne@chromium.org>
Cr-Commit-Position: refs/heads/master@{#613438}