Bug 852847 - Part 1: Disable browserElement_oop_KeyEvents.html.

In fact this test was already disabled; the OOP version was the same as
the in-process version because the test explicitly disabled OOP tabs.
Bug 852847 - Part 2: Do things later in our browser-element tests, thus avoiding doing things before the browser-element machinery has loaded.
Bug 852847 - Part 3: Simplify browserElementTestHelpers.js.
WIP: Move procesprioritymanager to main process.
No bug - s/bool/booleans/ in an error message in xpidl.py. r=me
More WIP
WIP: It compiles now.
Make ProcessPriorityManager compile.
Adding incomplete test_priority_oop_Simple test.