[Sync] Adjust UserEvent proto numbers, add Test message.

components/sync/protocol/user_event_specifics.proto

 // Copyright 2017 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.
 //
 // Sync protocol datatype extension for user events.
 
 // Update proto_value_conversions{.h,.cc,_unittest.cc} if you change
 // any fields in this file.
 
 syntax = "proto2";
@@ skipping 61 matching lines @@
   // Time of event, as measured on the client (unix epoch).
   optional int64 event_time_usec = 1;
 
   // The |global_id| field of the associated navigation, if there is one.
   optional int64 navigation_id = 2;
 
   // On startup we generate a new random id to identify which FieldTrialEvents
   // other events belonged to.
   optional fixed64 session_id = 3;
 
+  // 4-7 reserved for future scalars.
+
+  // Used for testing and debugging EventLog system.
+  message Test {}
+
   oneof event {
-    FieldTrialEvent field_trial_event = 4;
-    LanguageDetection language_detection = 5;
-    Translation translation = 6;
+    Test test = 8;
+    FieldTrialEvent field_trial_event = 9;
+    LanguageDetection language_detection = 10;
+    Translation translation = 11;
   }
 }