Fix dir-shadow-39.html.
The reference is just wrong. The spec says that unassigned nodes have no
computed styles (they have nothing to inherit from).
https://drafts.csswg.org/cssom/#dom-window-getcomputedstyle step 5:
If obj is not null, and elt is connected, part of the flat tree, and
...
Unassigned fallback content is not part of the flat tree, so we fail
that check.
Differential Revision: https://phabricator.services.mozilla.com/D220275
bugzilla-url: https://bugzilla.mozilla.org/show_bug.cgi?id=1908067
gecko-commit: f87404cfc945bcecb551c6f482053bc9ef157168
gecko-reviewers: smaug