Masayuki Nakano
9825ec9713
This will make the main patch simpler. However, some of them do not support `TreeKind::ShadowIncludingDOM` because it's unclear what should return as a parent node of a child of `ShadowRoot`, what should return as a child node of a shadow host. To avoid the developers to use them on wrong assumptions, this patch makes the users use `DOM`, `Flat` or `FlatForSelection` explicitly. Differential Revision: https://phabricator.services.mozilla.com/D281305