w3c/webappsec-referrer-policy#125
Default referrer policy was changed from no-referrer-when-downgrade to strict-origin-when-cross-origin.
But I could see there're many tests under "unset" value, and I think we have to update them to fallback to the new default.
For example:
https://github.com/web-platform-tests/wpt/blob/master/referrer-policy/gen/top.meta/unset/img-tag/cross-http.keep-origin.http.html