Add new policy: RestrictedManagedGuestSessionExtensionCleanupExemptList

A list of extension IDs that should be exempt from the reset procedure
proposed in the Imprivata shared workstation mode.

Bug: b:194485483
Change-Id: I4fc70c88a75bb687ac9806fb37281976e7fb69ae
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/3048193


Reviewed-by: Alexander Hendrich <hendrich@chromium.org>
Reviewed-by: Achuith Bhandarkar <achuith@chromium.org>
Commit-Queue: Maria Petrisor <mpetrisor@chromium.org>
Cr-Commit-Position: refs/heads/master@{#906349}