Bug 1642290: stop the process preallocator during normal shutdown, not post-CC r=nika

Post-CC is no longer needed given the landing of bug 1642491

Differential Revision: https://phabricator.services.mozilla.com/D79017

dom/ipc/PreallocatedProcessManager.cpp

@@ -101,8 +101,7 @@ PreallocatedProcessManagerImpl* PreallocatedProcessManagerImpl::Singleton() {
   if (!sSingleton) {
     sSingleton = new PreallocatedProcessManagerImpl;
     sSingleton->Init();
-    ClearOnShutdown(&sSingleton,
+    ClearOnShutdown(&sSingleton);
-                    ShutdownPhase::ShutdownPostLastCycleCollection);
   }
   return sSingleton;
   //  PreallocatedProcessManagers live until shutdown