Referrer Policy: Distinguish "default" from "no referrer when downgrade" [2/2]

We can drop a boolean from Document by adding a value to the ReferrerPolicy
enum to distinguish "default" from an explicitly-set "no referrer when
downgrade".

This patch also marginally increases the detail of the metrics we're
gathering ReferrerPolicy resets.

[1/2] Chromium: https://codereview.chromium.org/717503002/
[2/2] Blink: This patch.

BUG=431694

Review URL: https://codereview.chromium.org/698973008

git-svn-id: svn://svn.chromium.org/blink/trunk@185041 bbb929c8-8fbe-4397-9dbb-9b2b20218538