[] (3) Remove security checks on Document since there should now be no way to get to a Document from another origin.
Fixing https://www.w3.org/Bugs/Public/show_bug.cgi?id=23219
Affected topics: DOM APIs, HTML Syntax and Parsing, Security

git-svn-id: http://svn.whatwg.org/webapps@8273 340c8d12-0b0e-0410-8428-c7bf67bfef74