[giow] (3) Make the <form> past names map associations go away when the form owner changes, not when the element is removed from the document.
Fixing https://www.w3.org/Bugs/Public/show_bug.cgi?id=23073
Affected topics: DOM APIs

git-svn-id: http://svn.whatwg.org/webapps@8185 340c8d12-0b0e-0410-8428-c7bf67bfef74