[whatwg/fetch] Allow modifying the default referrer policy applied to certain fetches depending on engine-specific rules (#879)

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 `request`s 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.

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