Bug 1716727 - patchset #2 [2/2] Deexperimentalize stalling code r=glandium

Although further experiments and refinements are called for, the effect
of this patch has been overall significantly positive. Let it ride the
trains as-is for Fx 105, rather than blocking on those experiments.

(This patch conflicts with D153784, which will need to be rebased and
moved to a new Bugzilla bug.)

Depends on D153783

Differential Revision: https://phabricator.services.mozilla.com/D154839