In order to select elements referenced in idref/idreflist attributes,
we were querying the whole node document.
This is incorrect as id reference are scipes within a shadow root.
To fix this, we add a dedicated walker method to retrieve the referenced
node within the base node rootNode.
A test case is added to ensure we don't regress.
Differential Revision: https://phabricator.services.mozilla.com/D202451
