Switch xpcom/base/StaticPtr from NULL to nullptr, and add a necessary include.
Change ProcessPriorityManager's export directory.

It was mozilla/dom/ipc/ before, but this changes it to mozilla/, which matches the file's namespace.
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.
Add test_Preallocated.