[giow] (3) Make showModalDialog()'s returnValue logic more closely match Firefox. Previously it matched nobody. None of the browsers match each other. Firefox's behaviour seems the sanest.
Fixing https://www.w3.org/Bugs/Public/show_bug.cgi?id=21771
Affected topics: DOM APIs

git-svn-id: http://svn.whatwg.org/webapps@8008 340c8d12-0b0e-0410-8428-c7bf67bfef74