Also make sure that the pressure factor never gets to zero.

These new constants aren't deinitive in any way though I think that they are a bit more reasonable.

Depends on D67366

Differential Revision: https://phabricator.services.mozilla.com/D67367

[ghsync] From https://hg.mozilla.org/mozilla-central/rev/86167a71e7060d373e5ca4148e0e47ae5169d616