Use notification display service to collect persistent notifications.

chrome/browser/notifications/notification_test_util.h

Index: chrome/browser/notifications/notification_test_util.h
diff --git a/chrome/browser/notifications/notification_test_util.h b/chrome/browser/notifications/notification_test_util.h
index 5fbb3112f4d14d10670e663d68aeee0c6e0072fe..209dfde1bb421f6ad791a97e2c1df03449c673ae 100644
--- a/chrome/browser/notifications/notification_test_util.h
+++ b/chrome/browser/notifications/notification_test_util.h
@@ -50,6 +50,12 @@ class StubNotificationUIManager : public NotificationUIManager {
   // added to the Notification UI manager. Will be invoked on the UI thread.
   void SetNotificationAddedCallback(const base::Closure& callback);
 
+  // Emulates clearing a notification from the notification center
+  // without running any of the delegates. This is often possible
+  // from native notification centers.

[Peter Beverloo, 2016/12/02 13:43:34]

s/This is...notification/centers/ -->

This may happen when native notification centers don't inform us about closed
notifications, for example as a result of a system reboot.

[Miguel Garcia, 2016/12/06 12:01:18]

Done.

+  bool ClearNotificationById(const std::string& delegate_id,

[Peter Beverloo, 2016/12/02 13:43:34]

nit: what about "SilentDismissById" to further distinguish from CancelById? I'd
love the name to reflect how it's different from regular cancellation.

[Miguel Garcia, 2016/12/06 12:01:18]

Done.

+                             ProfileID profile_id);
+
   // NotificationUIManager implementation.
   void Add(const Notification& notification, Profile* profile) override;
   bool Update(const Notification& notification, Profile* profile) override;