Make //content responsible for generating notification Ids

content/browser/notifications/notification_database_data.proto

Index: content/browser/notifications/notification_database_data.proto
diff --git a/content/browser/notifications/notification_database_data.proto b/content/browser/notifications/notification_database_data.proto
index a3190338c966f3d112d581b6b1343b38844ec78d..3cdb8b6aed318d1d73e49f38099d04010a3659f1 100644
--- a/content/browser/notifications/notification_database_data.proto
+++ b/content/browser/notifications/notification_database_data.proto
@@ -11,9 +11,9 @@ 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;