[giow] (3) getSVGDocument() can return null (IDL fix).
Fixing https://www.w3.org/Bugs/Public/show_bug.cgi?id=25940
Affected topics: DOM APIs

git-svn-id: https://svn.whatwg.org/webapps@8814 340c8d12-0b0e-0410-8428-c7bf67bfef74