Currently, we allow extending selection range into a shadow of `<use>`
even though it's our own closed shadow. Therefore, we need to be
consistent about this in the each DOM API. I.e., we should treat the
shadow for SVG `<use>` as a part of the flat tree for selection.
Additionally, this makes `AbstractRange::RegisterSelection` fallible.
When the range boundaries are judged as disconnected, it's our bug,
but the crash report does not provide the enough information to us, and
it seems that not working selection may make users complain about that.
So, we could expect some tests would report the bug.
Differential Revision: https://phabricator.services.mozilla.com/D309988
Masayuki Nakano
deec723b5a
