It's only meaningfully read in two places:
* WebRender fallback code.
* Some widget DEBUG-only code, which on Linux is ifdef'd and on Windows does
some rather sketchy things like sleeping for 30ms
So I think it should be ok to remove, since WR fallback has its own flashing
pref as well, IIUC.
Differential Revision: https://phabricator.services.mozilla.com/D132313