ResourceRequest::clearHTTPReferrer should also clear the referrer policy

Also use the method in FrameLoader instead of setting an empty referrer

BUG=none
R=japhet@chromium.org

Review URL: https://codereview.chromium.org/150723002

git-svn-id: svn://svn.chromium.org/blink/trunk@166178 bbb929c8-8fbe-4397-9dbb-9b2b20218538