[benchmark] Do not abort() when readyState is DONE in destroyAllXHRs

This is to supress DevTools error messages, because on Chrome calling abort()
in an onreadystatechange handler in sync mode causes a NetworkError message,
even if it is after transition to DONE state.

R=tyoshino@chromium.org

Review URL: https://codereview.appspot.com/142550043

git-svn-id: http://pywebsocket.googlecode.com/svn/trunk@852 4ff78f4a-9131-11de-b045-6380ec9940d4