For all purposes, this is the same as devicePixelRatio. It was meant to
skip the resistFingerprinting check the devicePixelRatio getter does,
but we do that now using CallerType in WebIDL, so if we cared about that
for these tests (which we don't) we could just do
SpecialPowers.wrap(window).devicePixelRatio.
As a follow-up we could move the NoOverride to window for symmetry. But
it's only used by devtools touch simulation so not sure if worth it.
Differential Revision: https://phabricator.services.mozilla.com/D138021
b699e8cdc8