[CHROME] implement downgrade rules and fix https -> https redirects

Per the EFF ruleset,

https://www.eff.org/sites/all/themes/frontier/images/get-https-e-chrome.png

should be rewritten to

https://www.eff.org/sites/all/themes/frontier/images/got-https-e-chrome.png

But in the chrome version of HTTPS Everywhere, it isn't.

We also haven't implemented the "downgrade" rule attribute that allows rewrites to http, where those are required (although arguably because chrome blocks mixed content, this is much less useful than it is in Firefox).