Destroy CastMediaNotificationProvider when MediaNotificationService shuts down

CastMediaNotificationProvider depends on MediaRouter, which is a keyed
service. So the instance held by MediaNotificationService needs to be
destroyed in Shutdown().

Change-Id: Idf522c88838f2acc4116005865379b76428bbada
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1919754Reviewed-by: Tommy Steimel <steimel@chromium.org>
Commit-Queue: Takumi Fujimoto <takumif@chromium.org>
Cr-Commit-Position: refs/heads/master@{#716298}

chrome/browser/ui/global_media_controls/media_notification_service.cc

@@ -310,6 +310,12 @@ void MediaNotificationService::OnContainerDraggedOut(const std::string& id,
     observer.OnNotificationListChanged();
 }
 
+void MediaNotificationService::Shutdown() {
+  // |cast_notification_provider_| depends on MediaRouter, which is another
+  // keyed service.
+  cast_notification_provider_.reset();
+}
+
 void MediaNotificationService::OnOverlayNotificationClosed(
     const std::string& id) {
   // If the session has been destroyed, no action is needed.

chrome/browser/ui/global_media_controls/media_notification_service.h

@@ -74,6 +74,9 @@ class MediaNotificationService
   void OnContainerDestroyed(const std::string& id) override;
   void OnContainerDraggedOut(const std::string& id, gfx::Rect bounds) override;
 
+  // KeyedService implementation.
+  void Shutdown() override;
+
   // Called by the |overlay_media_notifications_manager_| when an overlay
   // notification is closed.
   void OnOverlayNotificationClosed(const std::string& id);