In this case we have a shadow hoot with display: contents, with no children.
Those children wouldn't be flattened tree children of the shadow host.
Instead of using the last light dom child and seek to it, use
FlattenedChildIterator's reverse iteration.
MozReview-Commit-ID: 18XL5Ong7ww
--HG--
extra : rebase_source : 2d34bd5b1fdd509a069ffa5052a7b7b3302be24c
Emilio Cobos Álvarez