Attempts to fetch the server cert.

    Returns:
      True if we succeed.

def _FetchServerCertificate(self):
    """Attempts to fetch the server cert.

    Returns:
      True if we succeed.
    """
    # Certificate is loaded and still valid.
    if self.server_certificate:
      return True

    response = self.http_manager.OpenServerEndpoint(
        "server.pem", verify_cb=self.VerifyServerPEM)

    if response.Success():
      self.server_certificate = response.data
      return True

    # We failed to fetch the cert, switch to slow poll mode.
    self.timer.SlowPoll()
    return False