Add new policy (DeviceUpdateStagingSchedule) for staging updates.

Add a new policy that will allow administrators to specify a schedule
for slowly updating their fleet.

BUG=chromium:858621

Change-Id: Ie21e538660a2d8d71c263a0f04397720820ce12a
Reviewed-on: https://chromium-review.googlesource.com/1119162
Commit-Queue: May Lippert <maybelle@chromium.org>
Reviewed-by: Pavol Marko <pmarko@chromium.org>
Cr-Original-Commit-Position: refs/heads/master@{#573954}
Cr-Mirrored-From: https://chromium.googlesource.com/chromium/src
Cr-Mirrored-Commit: b87cb494d1fe7fc76b40bc2739e63f6d5bcf1bc9
1 file changed