Bug 1283151 - make nsTimerEvent final; r=tbsaunde

Nothing should be deriving from nsTimerEvent, and perhaps this will make
some of the timer code slightly more efficient.

xpcom/threads/TimerThread.cpp

@@ -134,7 +134,7 @@ public:
 // This is a nsICancelableRunnable because we can dispatch it to Workers and
 // those can be shut down at any time, and in these cases, Cancel() is called
 // instead of Run().
-class nsTimerEvent : public CancelableRunnable
+class nsTimerEvent final : public CancelableRunnable
 {
 public:
   NS_IMETHOD Run() override;