Remove bogus FIXME from Document.idl
https://bugs.webkit.org/show_bug.cgi?id=98302

Reviewed by Adam Barth.

The FIXME claimed that document.body throwing an exception was not
specced, but in fact it is:
http://www.whatwg.org/specs/web-apps/current-work/multipage/dom.html#dom-document-body

* dom/Document.idl:


git-svn-id: http://svn.webkit.org/repository/webkit/trunk@130336 268f45cc-cd09-0410-ab3c-d52691b4dbfc