this doesn't belong in NotifierId because

a) message center doesn't know about profiles
b) it's only for chromeos/ash
c) it's pretty easy to forget to set (it's currently optional and we have a DCHECK in LoginStateNotificationBlocker to make sure it's set at the right times)

We should move the profile identifier into the AshMessageCenterController ShowClientNotification function. We should also be able to remove the centralized list of ash system notifiers that don't need a profile id, since it seems to only be there as a sanity check (see bullet point c above).