[MIX] Consider all CORS requests "active"

http://w3c.github.io/webappsec/specs/mixedcontent/#requirements-script

Currently, XHR & EventSource specifically fail when requesting insecure
content. This could be replaced with a general rule that CORS checks to
HTTP always fail. This also covers usually passive content that becomes
active via CORS, eg <img crossorigin>.

This means font requests to http would also fail, they could be given an
exception if needed.

In ServiceWorker, this means:

importScripts('http://...'); - fails
fetch('http://...'); - fails
cache.add('http://...'); - fails

…as they're all CORS dependant.

fetch('http://...', {mode: 'no-cors'});
cache.add(new Request('http://...', {mode: 'no-cors'}));

…these will give back a tainted response. Usual rules can apply if they're
used to satisfy requests to <script>, <img> etc.