Do serialization of computed border-spacing in stylo
Reland of D222204: This fixes an issues in
test_transitions_per_property.html where border-spacing was assumed
to serialize to 0px 0px. It follows the pattern used by other
properties that do not serialize this way.
Differential Revision: https://phabricator.services.mozilla.com/D227460
bugzilla-url: https://bugzilla.mozilla.org/show_bug.cgi?id=1918856
gecko-commit: 3c952710a4cd42c0b793e191e7ee0909430fd846
gecko-reviewers: emilio