[HTTPS-Everywhere] https everywhere in multiprocess Firefox

[yan]

In case there's anyone on this list who is using multiprocess Firefox
(aka e10s, aka the future of Firefox) nowadays, I've managed to get
HTTPS Everywhere to be e10s-compatible as far as I can tell. Testing in
this branch is appreciated:
https://github.com/EFForg/https-everywhere/pull/526

Tracking bugzilla bug: https://bugzilla.mozilla.org/show_bug.cgi?id=1014986

e10s will be on by default in Firefox sometime in the next year or so (I
think?), so it's a good idea for addon developers to start working on
compatibility.

-Yan