This is a BackupManifest schema change that will break recovering from
any backups created before this change. However, considering that we
haven't released this feature yet, I figured it wasn't worth the
effort of doing a schema version bump or handling of the prior
version.
I'm making the field optional in the BackupManifest because in the
event that we decide to backport the backup component to ESR 115,
we need to handle the fact that ESR 115 has no notion of a profile
group ID (and is unlikely to ever have such a notion).
Differential Revision: https://phabricator.services.mozilla.com/D218976
d23cddd9e6