Referrer Policy: Update <meta name="referrer"> values to match the spec
https://bugs.webkit.org/show_bug.cgi?id=137635

Reviewed by Jochen Eisinger.

Source/WebCore:

The Referrer Policy specification ([Working Draft][WD], [Editor's
draft[ED]) defines different keywords than we originally
implemented. We should support them in the interests of clarity and
interoperability with other browsers implementing the specification.

[WD]: http://www.w3.org/TR/referrer-policy/#referrer-policy-delivery-meta
[ED]: http://w3c.github.io/webappsec/specs/referrer-policy/#referrer-policy-delivery-meta

This patch is a port of Blink's https://codereview.chromium.org/607433002/

Tests: http/tests/security/referrer-policy-conflicting-policies.html
       http/tests/security/referrer-policy-https-no-referrer-when-downgrade.html
       http/tests/security/referrer-policy-https-no-referrer.html
       http/tests/security/referrer-policy-https-unsafe-url.html
       http/tests/security/referrer-policy-no-referrer-when-downgrade.html
       http/tests/security/referrer-policy-no-referrer.html

* dom/Document.cpp:
(WebCore::Document::processReferrerPolicy):

LayoutTests:

* http/tests/security/referrer-policy-https-no-referrer-expected.txt: Added.
* http/tests/security/referrer-policy-https-no-referrer-when-downgrade-expected.txt: Added.
* http/tests/security/referrer-policy-https-no-referrer-when-downgrade.html: Added.
* http/tests/security/referrer-policy-https-no-referrer.html: Added.
* http/tests/security/referrer-policy-https-unsafe-url-expected.txt: Added.
* http/tests/security/referrer-policy-https-unsafe-url.html: Added.
* http/tests/security/referrer-policy-invalid-expected.txt: Update the error message with new keywords.
* http/tests/security/referrer-policy-no-referrer-expected.txt: Added.
* http/tests/security/referrer-policy-no-referrer-when-downgrade-expected.txt: Added.
* http/tests/security/referrer-policy-no-referrer-when-downgrade.html: Added.
* http/tests/security/referrer-policy-no-referrer.html: Added.


git-svn-id: http://svn.webkit.org/repository/webkit/trunk@174640 268f45cc-cd09-0410-ab3c-d52691b4dbfc