SABs become foreground-finalizable so that we can access the runtime
during finalization.  Then a simple counter on the runtime will track
live SABs for the runtime, and the predicate on the context can get
its information from the runtime.

Fallout: SABs are now enabled on the globals used for jsapi-tests.

Differential Revision: https://phabricator.services.mozilla.com/D55783

--HG--
extra : moz-landing-system : lando