Bug 844323 - Prelude part 4: Use 's' prefix on class-static variables, not 'g'. r=me
Bug 844323 - Part 1: Move process preallocation logic out of ContentParent and into a new file, PreallocatedProcessManager. r=bent

Also make the PreallocatedProcessManager respond to pref changes.  This allows us to write a test involving the preallocated process.  Making this change was the main motivation for this patch; I moved the logic out of ContentParent because with the pref-watching code it was becoming unweildy.
Bug 844323 - Part 2 (The Main Event): Move ProcesPriorityManager to the main process. r=bent,khuey
Bug 844323 - Part 3a: Adding promises to browserElementTestHelpers.js. r=khuey,nsm
Bug 844323 - Part 3b: Adding process-priority testcases. r=khuey,nsm
Bug 860799 - Part 1: Give frames expecting a system message priority at least BACKGROUND_PERCEIVABLE, so long as either haven't yet handled the system message or are holding the CPU/high-priority wake lock. r=khuey
Bug 860799 - Part 2: Test that we give frames expecting a system message priority above vanilla BACKGROUND. r=khuey
Bug 861441 - When changing a proces's CPU priority, call setpriority() on all threads, not just the main thread. r=gsvelto

It turns out that setpriority() on Linux sets the priority of just one
thread.  If you call setpriority() on a pid, you're setting the priority
of only the main thread in that process.
Bug 844323 - Follow-up: Fix typo in BrowserElementChildPreload that was breaking bg --> fg transitions in childVisibilityChanged.

Also modify the test so it catches this mistake.
Bug 852847 - WIP: Part 4: Try to fix the SetVisibleFrames{,2} tests.