Called on the IO thread when the browser needs credentials from the user. |isProxy| indicates whether the host is a proxy server. |host| contains the hostname and |port| contains the port number. Return true (1) to continue the request and call cef_auth_callback_t::cont() when the authentication information is available. If the request has an associated browser/frame then returning false (0) will result in a call to GetAuthCredentials on the cef_request_handler_t associated with that browser, if any. Otherwise, returning false (0) will cancel the request immediately. This function will only be called for requests initiated from the browser process.