[giow] (1) dialog::backdrop needs positioning, it isn't _that_ magical
Fixing https://www.w3.org/Bugs/Public/show_bug.cgi?id=23298
Affected topics: Rendering

git-svn-id: http://svn.whatwg.org/webapps@8197 340c8d12-0b0e-0410-8428-c7bf67bfef74