Not really a fan of this, but I can't think of a better alternative
really... Ideas welcome :)

The main issue is that in bug 1662975 we made window.print() not return
until the user has closed the print / print preview dialog (as it is
needed for some use cases). This matches other browsers, too.

We use an nsAutoSyncOperation here, in order not to violate the
run-to-completion invariants, which turns off micro-tasks, timers,
etc... However we'd still want promises inside mozPrintCallback to
resolve and such, which is a bit contradictory. It is really awkward to
have this behavior change based on whether we have a print callback...

Differential Revision: https://phabricator.services.mozilla.com/D89298