Bug 1851045 - Part 5: Eliminate nsThreadManager's static destructor, r=emilio

This both reduces the number of registered static destructors, and avoids the
assertion in the LinkedList destructor without depending on a StaticAutoPointer
(which requires an allocation & additional indirection) by using the new
NeverDestroyed type from part 4.

Differential Revision: https://phabricator.services.mozilla.com/D189249

xpcom/threads/nsThreadManager.cpp

@@ -20,6 +20,7 @@
 #include "mozilla/EventQueue.h"
 #include "mozilla/InputTaskManager.h"
 #include "mozilla/Mutex.h"
+#include "mozilla/NeverDestroyed.h"
 #include "mozilla/Preferences.h"
 #include "mozilla/ProfilerMarkers.h"
 #include "mozilla/SpinEventLoopUntil.h"
@@ -257,8 +258,8 @@ NS_IMPL_CI_INTERFACE_GETTER(nsThreadManager, nsIThreadManager)
 //-----------------------------------------------------------------------------
 
 /*static*/ nsThreadManager& nsThreadManager::get() {
-  static nsThreadManager sInstance;
-  return sInstance;
+  static NeverDestroyed<nsThreadManager> sInstance;
+  return *sInstance;
 }
 
 nsThreadManager::nsThreadManager()

xpcom/threads/nsThreadManager.h

@@ -18,6 +18,9 @@ namespace mozilla {
 class IdleTaskManager;
 class SynchronizedEventQueue;
 class TaskQueue;
+
+template <typename T>
+class NeverDestroyed;
 }  // namespace mozilla
 
 class BackgroundEventTarget;
@@ -103,6 +106,8 @@ class nsThreadManager : public nsIThreadManager {
   }
 
  private:
+  friend class mozilla::NeverDestroyed<nsThreadManager>;
+
   nsThreadManager();
 
   nsresult SpinEventLoopUntilInternal(