[giow] (3) Change iframe, frame, and object.contentDocument to return null when cross-origin documents are involved.
Fixing https://www.w3.org/Bugs/Public/show_bug.cgi?id=23219
Affected topics: HTML

git-svn-id: http://svn.whatwg.org/webapps@8272 340c8d12-0b0e-0410-8428-c7bf67bfef74