No other property returns `currentcolor` in the computed style, and
Chrome agrees with us on this test.

Differential Revision: https://phabricator.services.mozilla.com/D120725

bugzilla-url: https://bugzilla.mozilla.org/show_bug.cgi?id=1722031
gecko-commit: 2a0274aa40148b9d89d79d844aa27b3ae2430bd8
gecko-reviewers: mstange