…ions.

See the extended commit message for the following spec change:

  w3c/csswg-drafts@6b36d41

Basically, by failing to take the absolute value, for certain content we can end
up doing division by zero which will mean that the test included in this patch
will cause an assertion to fail in debug builds and return
"matrix(NaN, NaN....)" in release builds.

Differential Revision: https://phabricator.services.mozilla.com/D9618