[HTTPS-Everywhere] Mixed Content Blocker

[Jacob Hoffman-Andrews]

> 1. We handle both cases by adding platform="mixedcontent" and restricting
> the rules to type 3 browsers.
> 2. If type 2 browsers became available, there would be no easy way of
> telling which rules should be enabled on those browsers, nor would there be
> any way of actually doing so (while keeping those rules disabled on type 1
> browsers).

Rules with platform="mixedcontent" should be enabled on any future type 2 browsers.

In general, people are not writing new "mixedcontent" rules and adding them to
HTTPSEverywhere. The mixedcontent platform is a way of disabling rules that used
to work, and will work again if FF and Chrome implement the type 2 rules you describe.