Adding UMA for persistent notifications.

content/public/common/persistent_notification_status.h

Index: content/public/common/persistent_notification_status.h
diff --git a/content/public/common/persistent_notification_status.h b/content/public/common/persistent_notification_status.h
index b8f14094b3a5203e5c355cad5b66be397adf22ee..38d1dab4f2aa9752b19bafe342268a95ced985c5 100644
--- a/content/public/common/persistent_notification_status.h
+++ b/content/public/common/persistent_notification_status.h
@@ -8,6 +8,7 @@
 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,
@@ -25,6 +26,9 @@ enum PersistentNotificationStatus {
   // 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