[giow] (1) Make non-modal <dialog>s use the regular containing block, since they're not on the top layer.
Fixing https://www.w3.org/Bugs/Public/show_bug.cgi?id=20418
Affected topics: CSS, DOM APIs, HTML

git-svn-id: http://svn.whatwg.org/webapps@8448 340c8d12-0b0e-0410-8428-c7bf67bfef74