Bug 1689953: Harmonize shutdown phase definitions across nsTerminator and AppShutdown r=dthayer,chutten

This patch wants to solve several quirks around the shutdown terminator.

 - Use the same shutdown phase definitions in AppShutdown and nsTerminator. This touches quite a few files.
 - Ensure that the terminator phase shift is handled before any shutdown observer notifications are sent and reduce its heartbeat duration.
 - Add missing phases to the shutdown telemetry.

Please note that this changes the unit of "tick" to 100ms rather than 1s.
As a side effect, we also remove the obsolete "shutdown-persist" context.

While the existing test coverage continues to prove the most important functions, we acknowledge the wish for better test coverage with [[ https://bugzilla.mozilla.org/show_bug.cgi?id=1693966 | bug 1693966 ]].

Differential Revision: https://phabricator.services.mozilla.com/D103626

toolkit/components/extensions/webrequest/ChannelWrapper.cpp

@@ -100,7 +100,7 @@ static LinkedList<ChannelWrapper>& ChannelList() {
   static UniquePtr<ChannelListHolder> sChannelList;
   if (!sChannelList) {
     sChannelList.reset(new ChannelListHolder());
-    ClearOnShutdown(&sChannelList, ShutdownPhase::Shutdown);
+    ClearOnShutdown(&sChannelList, ShutdownPhase::XPCOMShutdown);
   }
   return *sChannelList;
 }