Fix cross-domain XHR on IE9 http://codereview.appspot.com/6622053
Fix cross-domain XHR on IE9
http://codereview.appspot.com/6622053

XMLHttpRequest on IE9 does not allow cross-domain requests and requires
the use of XDomainRequest for this purpose.  This change tests if the
browser we're on doesn't support cors and fallbacks on XDomainRequest in
that case.

R=metaweta@gmail.com


git-svn-id: eb21526166532bd3afc2c1499c0cf105ce815f5a@5083 360991c0-9f3b-0410-ba87-8b362a00edf4

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