Bug 838616 - Boost preallocated process's priority sooner after we decide to transform it into an app process. r=cjones, a=tef+
Bug 838616 - Eliminate a process-priority race condition around preallocated process creation. r=cjones, a=tef+

Previously, it was possible to return a dead preallocated process.  This
patch eliminates or at least significantly reduces the likelihood of
this race.