Issue 71531 - chromium - Specifying port in permissions when doing a chrome extension leads to broken XHR behavior - An open-source browser project to help move the web forward. - Google Project Hosting http://code.google.com/p/chromium/issues/detail?id=71531
これにハマった。XHR の readyState が 4 で status が 0 で返ってくる。これは分かりづらい仕様だ。