[xhr_event_logger] Add an option to test sync XHR
[xhr_event_logger] Add an option to test sync XHR

R=hiroshige@chromium.org

Review URL: https://codereview.appspot.com/139550043
GoogleChromeLabs/pywebsocket3,google/pywebsocket,GoogleChromeLabs/pywebsocket3,GoogleChromeLabs/pywebsocket3,googlearchive/pywebsocket,googlearchive/pywebsocket,google/pywebsocket,google/pywebsocket,googlearchive/pywebsocket