Sends cross origin headers.

def cross_origin_headers(self):
        """Sends cross origin headers."""
        if not self.is_cross_origin():
            return False
        # we allow everything
        self.send_header("Access-Control-Allow-Methods",
                         "GET, POST, PUT, DELETE, HEAD")
        allow_headers = _getheader(self.headers,
                                   'access-control-request-headers')
        if allow_headers is not None:
            self.send_header("Access-Control-Allow-Headers", allow_headers)
        self.send_header("Access-Control-Allow-Origin", "*")
        self.send_header("Access-Control-Allow-Credentials", "true")
        return allow_headers is not None