When you're in a ShadowRoot and can share style with a sibling, the sharing code
is smart enough to skip document author rules.

But then it could get confused if you also include document rules, since
revalidation selectors are matched against these.

This is not a correctness issue, because we're matching more than what we need,
and avoid sharing if we failed.

Also fix the detection for user rules in any_applicable_rule_data.