Multiple content processes in Nightly

Hello everyone, We've been working on the e10s-multi project for a while now and are looking at turning on multiple content processes in Nightly. Our plan is to start with two content processes (compared to the single content process we currently use) and if that goes well, we'll start ramping up the number of content processes we use as well as playing with more exciting process allocation strategies. We are not yet planning to ride the trains. In order to turn on in Nightly, Gabor Krizsanits has been doing a ton of work to make sure our tests are green. We've had to disable a couple of them in e10s mode and for others we've been forcing them to use a single content process (to be clear, the tests themselves are broken with multiple content processes and the underlying code is not). We will be working on fixing the tests as we can as well as turning the disabled tests back on [1]. We have two known bugs that we're enabling with: * Service workers for the same origin can run simultaneously in multiple processes [2]. We expect the user-visible aspects of this bug to be limited to desktop notifications being duplicated (for each content process that has a bogus service worker running in it). bkelly is leading a team to fix this. * DOM storage doesn't properly propagate changes to other processes [3]. This could cause web sites to misbehave. janv is working on fixing this. Let Gabor or me know if you have any concerns or comments and needinfo us on bugs that you run into. [1] https://bugzilla.mozilla.org/show_bug.cgi?id=1315042 [2] https://bugzilla.mozilla.org/show_bug.cgi?id=1231208 [3] https://bugzilla.mozilla.org/show_bug.cgi?id=666724 -- Blake