Bug 852847 - Part 2: Do things later in our browser-element tests, thus avoiding doing things before the browser-element machinery has loaded.

Also set network.disable.ipc.security to true and leave it that way. This prevents security errors in the tests which happen when we pop the pref.
Bug 852847 - Part 3: Re-enable some browser-element tests.
Bug 844323 - WIP: Move procesprioritymanager to main process.

Don't forget to remove ProcessPriorityManager-old.cpp
Bug 844323 - ProcessPriorityManager tests.