Allow modifying the default referrer policy applied to certain fetches depending on engine-specific rules

[ehsan]

Step 2.6 of Main fetch (https://fetch.spec.whatwg.org/#main-fetch) currently mandates the browser to use the "no-referrer-when-downgrade" referrer policy when nothing else sets the policy. It would be nice to add a step prior to this to allow the engine to run any engine-specific rules that may apply to the request which would allow the engine to infer a referrer policy for it.

The use case for this would be to specify the current behaviour of shipping engines such as Firefox (since version 59) which has been applying a default policy of strict-origin-when-cross-origin to third-party requests in private windows, as well as current versions of Safari which I believe do something similar for third-party origins with potential tracking capabilities (per https://webkit.org/blog/8311/intelligent-tracking-prevention-2-0/, CCing @johnwilander who would know more about that.) Firefox is interested to try to ship a similar feature to the Safari feature mentioned here too.

[annevk]

It seems it should allow for "no-referrer-when-downgrade" or stricter, but not say "unsafe-url".