Bug 1384034 - Label UpdateTimerCallback in ServiceWorkerManager. r=bkelly

Label with SystemGroup because UpdateTimerCallback only sends an IPC message
to the parent without touching any web contents.

dom/workers/ServiceWorkerManager.cpp

@@ -34,6 +34,7 @@
 #include "mozilla/ClearOnShutdown.h"
 #include "mozilla/ErrorNames.h"
 #include "mozilla/LoadContext.h"
+#include "mozilla/SystemGroup.h"
 #include "mozilla/Telemetry.h"
 #include "mozilla/dom/BindingUtils.h"
 #include "mozilla/dom/ContentParent.h"
@@ -4312,6 +4313,10 @@ ServiceWorkerManager::ScheduleUpdateTimer(nsIPrincipal* aPrincipal,
     return;
   }
 
+  // Label with SystemGroup because UpdateTimerCallback only sends an IPC message
+  // (PServiceWorkerUpdaterConstructor) without touching any web contents.
+  timer->SetTarget(SystemGroup::EventTargetFor(TaskCategory::Other));
+
   nsCOMPtr<nsITimerCallback> callback = new UpdateTimerCallback(aPrincipal,
                                                                 aScope);