Bug 1458383 - Don't dispatch mediaBlockStop notification. r=baku

This is a partial backout of Bug 1347791 part 3; a5fbb7e2d1d0.

We don't need to track in the front end code when a tab is foregrounded,
so we don't need to dispatch this message any more.

MozReview-Commit-ID: 6M0n9Ik65vE

--HG--
extra : rebase_source : 37ca839bdf9f323b11e010df643c31f895c602f5

dom/audiochannel/AudioChannelService.cpp

@@ -791,25 +791,9 @@ AudioChannelService::AudioChannelWindow::RemoveAgent(AudioChannelAgent* aAgent)
 void
 AudioChannelService::AudioChannelWindow::NotifyMediaBlockStop(nsPIDOMWindowOuter* aWindow)
 {
-  // Can't use raw pointer for lamba variable capturing, use smart ptr.
-  nsCOMPtr<nsPIDOMWindowOuter> window = aWindow;
-  NS_DispatchToCurrentThread(NS_NewRunnableFunction(
-    "dom::AudioChannelService::AudioChannelWindow::NotifyMediaBlockStop",
-    [window]() -> void {
-      nsCOMPtr<nsIObserverService> observerService =
-        services::GetObserverService();
-      if (NS_WARN_IF(!observerService)) {
-        return;
-      }
-
-      observerService->NotifyObservers(ToSupports(window),
-                                       "audio-playback",
-                                       u"mediaBlockStop");
-    })
-  );
-
   if (mShouldSendActiveMediaBlockStopEvent) {
     mShouldSendActiveMediaBlockStopEvent = false;
+    nsCOMPtr<nsPIDOMWindowOuter> window = aWindow;
     NS_DispatchToCurrentThread(NS_NewRunnableFunction(
       "dom::AudioChannelService::AudioChannelWindow::NotifyMediaBlockStop",
       [window]() -> void {