Adding UMA for persistent notifications.

content/public/common/persistent_notification_status.h

 // Copyright 2014 The Chromium Authors. All rights reserved.
 // Use of this source code is governed by a BSD-style license that can be
 // found in the LICENSE file.
 
 #ifndef CONTENT_PUBLIC_COMMON_PERSISTENT_NOTIFICATION_STATUS_H_
 #define CONTENT_PUBLIC_COMMON_PERSISTENT_NOTIFICATION_STATUS_H_
 
 namespace content {
 
 // Delivery status for persistent notification clicks to a Service Worker.
+// PersistentNotificationStatus entries should not be reordered or removed.
 enum PersistentNotificationStatus {
   // The notificationclick event has been delivered successfully.
   PERSISTENT_NOTIFICATION_STATUS_SUCCESS = 0,
 
   // The event could not be delivered because the Service Worker is unavailable.
   PERSISTENT_NOTIFICATION_STATUS_NO_SERVICE_WORKER,
 
   // The event could not be delivered because of a Service Worker error.
   PERSISTENT_NOTIFICATION_STATUS_SERVICE_WORKER_ERROR,
 
   // The event has been delivered, but the developer extended the event with a
   // promise that has been rejected.
   PERSISTENT_NOTIFICATION_STATUS_EVENT_WAITUNTIL_REJECTED,
 
   // The event could not be delivered because the data associated with the
   // notification could not be read from the database.
   PERSISTENT_NOTIFICATION_STATUS_DATABASE_ERROR,
+
+  // Only add new entries above this line.
+  PERSISTENT_NOTIFICATION_STATUS_MAX
 };
 
 }  // content
 
 #endif  // CONTENT_PUBLIC_COMMON_PERSISTENT_NOTIFICATION_STATUS_H_