[giow] (3) Fix broken logic around IDL attributes for form submission.

Fixing https://www.w3.org/Bugs/Public/show_bug.cgi?id=17185
Affected topics: HTML

git-svn-id: http://svn.whatwg.org/webapps@7648
340c8d12-0b0e-0410-8428-c7bf67bfef74

Fixes W3C bug: https://www.w3.org/Bugs/Public/show_bug.cgi?id=18191