Support withCredentials attribute for XHR requests (Chrome 3+, Firefox 3.5+, Opera12+, Safari 4+, Internet Explorer 10+)

Review at http://gwt-code-reviews.appspot.com/1879804

Review by: cromwellian@google.com

git-svn-id: https://google-web-toolkit.googlecode.com/svn/trunk@11471 8db76d5a-ed1c-0410-87a9-c151d255dfc7