• url String - HTTP 的 Referrer 地址（HTTP源地址）。
• policy String - 可以设置为 default, unsafe-url, no-referrer-when-downgrade, no-referrer, origin, strict-origin-when-cross-origin, same-origin 和 strict-origin. 前往 Referrer-Policy spec 可以查看这些policy的定义。

Referrer Object

• url String - HTTP Referrer URL.
• policy String - Can be default, unsafe-url,
  no-referrer-when-downgrade, no-referrer, origin,
  strict-origin-when-cross-origin, same-origin or strict-origin.
  See the Referrer-Policy spec for more details on the
  meaning of these values.