Bug 844323 - Prelude part 2: Switch xpcom/base/StaticPtr from NULL to nullptr, and add a necessary include.

This is necessary to make this header compile in one of the places I use it.  Anyway it's the right change.
Bug 844323 - Prelude part 3: Change ProcessPriorityManager's export directory.

It was mozilla/dom/ipc/ before, but this changes it to mozilla/, which matches the class's namespace.
Bug 844323 - Part 1: Move process preallocation logic out of ContentParent and into a new file, PreallocatedProcessManager.

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.
Bug 844323 - Part 3: ProcessPriorityManager tests.
Bug 847592 - Part 1: Add an additional lowCPUPriority argument to hal::SetProcessPriority.
Bug 847592 - Part 2: When a process has priority FOREGROUND_HIGH, renice all other processes.
Bug 847592 - Tests for renicing all other processes when one process gets priority FOREGROUND_HIGH