document.referrer should reflect Referer / referrer policy better

[annevk]

E.g., if you have a no-referrer policy and create an initial about:blank popup, that should not have the creator URL as its document's referrer.

[domfarolino]

/cc @hiroshige-g

[domfarolino]

Related: I added a WPT at https://github.com/web-platform-tests/wpt/blob/master/referrer-policy/generic/inheritance/iframe-inheritance-about-blank.html asserting that an about:blank iframe inherit its creator document's full URL, unredacted by its referrer policy, as its document.referrer. So if we make referrer policy inheritance a thing, we'll need to change that test's expectations I believe.