Make //content responsible for generating notification Ids

content/browser/notifications/notification_database_data.proto

 // Copyright 2015 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.
 
 syntax = "proto2";
 
 option optimize_for = LITE_RUNTIME;
 
 package content;
 
 // Stores information about a Web Notification. This message is the protocol
 // buffer meant to serialize the content::NotificationDatabaseData structure.
 //
-// Next tag: 5
+// Next tag: 6
 message NotificationDatabaseDataProto {
-  optional int64 notification_id = 1;
+  optional string notification_id = 5;
 
   optional string origin = 2;
   optional int64 service_worker_registration_id = 3;
 
   // A notification action, corresponds to content::PlatformNotificationAction.
   //
   // Next tag: 6
   message NotificationAction {
     // Corresponds to PlatformNotificationActionType.
     enum Type {
@@ skipping 31 matching lines @@
     optional int64 timestamp = 12;
     optional bool renotify = 13;
     optional bool silent = 7;
     optional bool require_interaction = 11;
     optional bytes data = 8;
     repeated NotificationAction actions = 10;
   }
 
   optional NotificationData notification_data = 4;
 }