Wes gave me an r+ on IRC, so I landed this: https://hg.mozilla.org/integration/mozilla-inbound/rev/c6371662e21e
Backed out in https://hg.mozilla.org/integration/mozilla-inbound/rev/e4861bc9f86f - other than the orange in mochitests 1-3 and 6-8, robocop, crashtests and reftests, everything when swimmingly with that push.
I pushed to try to make sure this patch wasn't the guilty one. Re-landing: https://hg.mozilla.org/integration/mozilla-inbound/rev/526235473719
Uplifted to aurora as part of a roll-up patch: https://hg.mozilla.org/releases/mozilla-aurora/rev/2fb0a358eaf6