Build an ACLToken from the request.

def _BuildToken(self, request, execution_time):
    """Build an ACLToken from the request."""
    token = access_control.ACLToken(
        username=request.user,
        reason=request.args.get("reason", ""),
        process="GRRAdminUI",
        expiry=rdfvalue.RDFDatetime.Now() + execution_time)

    for field in ["Remote_Addr", "X-Forwarded-For"]:
      remote_addr = request.headers.get(field, "")
      if remote_addr:
        token.source_ips.append(remote_addr)
    return token