RE-add the ability to delete notification ids unknown by the display service.

content/browser/notifications/notification_database.cc

Index: content/browser/notifications/notification_database.cc
diff --git a/content/browser/notifications/notification_database.cc b/content/browser/notifications/notification_database.cc
index 192368204773d8b8825a8e133649c52473c786f3..677a3270ef1a005c491f5768f0856ed3ef3fbe5a 100644
--- a/content/browser/notifications/notification_database.cc
+++ b/content/browser/notifications/notification_database.cc
@@ -328,11 +328,6 @@ NotificationDatabase::ReadAllNotificationDataInternal(
       continue;
     }
 
-    // Silently ignore the notification if it doesn't have an ID assigned.
-    // TODO(peter): Remove this clause when Chrome 55 has branched.
-    if (notification_database_data.notification_id.empty())
-      continue;
-
     notification_data_vector->push_back(notification_database_data);
   }