[giow] (0) Make modal <dialog> ancestors inert too.
Fixing https://www.w3.org/Bugs/Public/show_bug.cgi?id=23456
Affected topics: HTML

git-svn-id: http://svn.whatwg.org/webapps@8343 340c8d12-0b0e-0410-8428-c7bf67bfef74