This blocks FOG initialization on notifying the user. This **does**
change the existing functionality, even when the Messaging System is
not used to notify the user: in legacy situations, this will display a
background tab and the promise will resolve quickly (not immediately),
falling back to an infobar that might be delayed 10-60s before being
displayed and being resolved (again, not immediately).
(Legacy Telemetry was blocked already, because the existing user
notification blocks sending Legacy Telemetry.)
Finally, the default browser prompt is blocked so that there's no race
between the (potentially blocking) Messaging System notification and
other prompts. This also changes the behaviour by allowing the
Messaging System notification to be resolved and a subsequent
prompt (such as the default browser prompt) to be displayed
immediately afterward.
Differential Revision: https://phabricator.services.mozilla.com/D236624
4773306fc7