Summary:
FlushTarget wants to decide whether we should flush the parent document or all
of them. However, the only point of flushing parent documents is that media
query changes could affect the document we really want to flush.
That's completely pointless if we actually don't flush the subdocument, so just
skip doing that. This case is already checked (see the DocumentNeedsRestyle
stuff, which is also somewhat poorly named, which walks up the document chain).
Reviewers: xidorn
Bug #: 1443483
Differential Revision: https://phabricator.services.mozilla.com/D682
MozReview-Commit-ID: LiI7IrUBeqq
41e05df804