Store the referrer policy used for a request in the request.

BUG=334125
R=abarth@chromium.org, japhet@chromium.org

Review URL: https://codereview.chromium.org/131783005

git-svn-id: svn://svn.chromium.org/blink/trunk@165916 bbb929c8-8fbe-4397-9dbb-9b2b20218538