This ended up not being so small of a patch as I'd have thought, since it
propagated a bit. But most of it is mechanical. Interesting part is
NonNegativeNumberOrPercentage and the actual uses of the NonNegative stuff and
during parsing.

This looks like it'd fix a few correctness issues during interpolation for all
the types except for BorderRadius and co (which handled it manually).

I should write tests for those in a different patch.

Differential Revision: https://phabricator.services.mozilla.com/D14673