CSP 1.1: Implement 'referrer' directive.

Spec: http://w3c.github.io/webappsec/specs/content-security-policy/csp-specification.dev.html#referrer

This patch does not implement interaction with <meta referrer>-specified
referrer policies. That work will come in a future patch.

BUG=309551

Review URL: https://codereview.chromium.org/138913005

git-svn-id: svn://svn.chromium.org/blink/trunk@165450 bbb929c8-8fbe-4397-9dbb-9b2b20218538