Let the @scope root search cross the shadow boundary if ::part selector is used.
This allows the scope search to extend beyond the shadow tree the element
is in any number of times, which is required to get to the DOM tree defining
the ::part selector in question. If any shadow DOM tree in the chain does not
use exportparts, it is rejected when the style rule selector is matched.
Differential Revision: https://phabricator.services.mozilla.com/D218256
bugzilla-url: https://bugzilla.mozilla.org/show_bug.cgi?id=1908279
gecko-commit: 13085ea1774da11e805086aa28cb7c77223a5d26
gecko-reviewers: firefox-style-system-reviewers, emilio