[giow] (3) Add getSVGDocument() to object and iframe also, with the same definition as for embed.
Fixing https://www.w3.org/Bugs/Public/show_bug.cgi?id=25940

git-svn-id: https://svn.whatwg.org/webapps@8811 340c8d12-0b0e-0410-8428-c7bf67bfef74