Fix an assertion in selector-matching.
We can have combinator sequences like [>, ], and they are fine.
Add a test to make sure they're handled correctly.
Differential Revision: https://phabricator.services.mozilla.com/D208668
bugzilla-url: https://bugzilla.mozilla.org/show_bug.cgi?id=1891296
gecko-commit: c6154fb40c2e313b676d438b2eb9851118e9930b
gecko-reviewers: dshin