ReferrerPolicy:
    | "no-referrer"
    | "no-referrer-when-downgrade"
    | "origin"
    | "origin-when-cross-origin"
    | "same-origin"
    | "strict-origin"
    | "strict-origin-when-cross-origin"
    | "unsafe-url"

Referrer policy for the iframe.

User documentation for the policies can be found here: https://developer.mozilla.org/en-US/docs/Web/API/HTMLIFrameElement/referrerPolicy