Make sure to use the correct referrer policy in the FrameFetchContext

Before, we might have used the referrer from the request, but the policy
from the document. Since when then didn't update the request, an invalid
referrer might have slipped.

BUG=407510
R=mkwst@chromium.org

Review URL: https://codereview.chromium.org/534233002

git-svn-id: svn://svn.chromium.org/blink/trunk@181317 bbb929c8-8fbe-4397-9dbb-9b2b20218538