Fix at-property-animation to account for spec ambiguity.
See w3c/csswg-drafts#10371 for the spec issue.
Differential Revision: https://phabricator.services.mozilla.com/D211871
bugzilla-url: https://bugzilla.mozilla.org/show_bug.cgi?id=1899318
gecko-commit: a869d0eda92c3cb0ae54522343e9079f627dfc25
gecko-reviewers: firefox-style-system-reviewers, zrhoffman