Bug 1819497 - Don't race on static bool for initialization. r=gfx-reviewers,aosmond

We could do non-racy static init here (e.g. with a static initializer
self-calling-closure), but there doesn't seem to be a strong reason for
this. Let's just use a switch and get robustness from -Werror=switch.

Differential Revision: https://phabricator.services.mozilla.com/D188054