[giow] (2) Work on dir=auto. More work may come.
Fixing https://www.w3.org/Bugs/Public/show_bug.cgi?id=17835
Affected topics: HTML

git-svn-id: http://svn.whatwg.org/webapps@7863 340c8d12-0b0e-0410-8428-c7bf67bfef74

No objections on public-html, so taking this additional cherry-pick from Week 18 changes.