During an HttpCase, when the Chrome browser starts, it may happens that
the websocket is unreachable. In such a case, the stop method is called
and tries to stop the browser gracefully by sending a `Browser.close`
through the websocket ... which is not reachable.
The result is a confusing traceback. With this commit, nothing is sent
is the websocket is not available.
Moreover, the `sigxcpu_handler` attribute is used in the stop method but
not yet declared. Fixed by moving it before calling the _chrome_start
method.
X-original-commit: 0d156fd3bc114b40680ca5aa0456e0f21a5b8cff
Christophe Monniez
426fc163ba