In order to get better shutdown hang reporting, we want to distinguish also the last phases, namely `XPCOMShutdownThreads`, `XPCOMShutdownMainThread` and `CCPostLastCycleCollection`. This also makes `XPCOMShutdownNotified()` obsolete and we need to slightly re-arrange the watchdog function. Differential Revision: https://phabricator.services.mozilla.com/D145433