Align '<meta name="referrer" content="invalid">' behavior with CSP 1.1.

CSP 1.1 specifies that a 'referrer' directive with an invalid value
fails closed by setting the Document's referrer policy to 'never'[1].
This behavior conflicts with the <meta> definition on the WHATWG
wiki[2].

This patch resolves the conflict by preferring CSP's behavior. Since we
didn't have any tests for our behavior in the case of an invalid
referrer, I suppose that's acceptable. :)

Additionally, this patch adds a counter for sites which change their
referrer policies on the fly. That behavior is explicitly allowed by the
<meta> spec, and explicitly disallowed by the CSP spec. Depending
on usage in the wild, it would be nice to change one or the other of
these specifications.

[1]: http://w3c.github.io/webappsec/specs/content-security-policy/csp-specification.dev.html#referrer
[2]: http://wiki.whatwg.org/wiki/Meta_referrer#Specification

Review URL: https://codereview.chromium.org/143983011

git-svn-id: svn://svn.chromium.org/blink/trunk@165627 bbb929c8-8fbe-4397-9dbb-9b2b20218538