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

components/sync/protocol/user_event_specifics.proto

Index: components/sync/protocol/user_event_specifics.proto
diff --git a/components/sync/protocol/user_event_specifics.proto b/components/sync/protocol/user_event_specifics.proto
index e2af33ac9cd9a393acff510e64c6b51d9888aacc..7a3aab7d41286c79b3b2f4f11a038038562d66ac 100644
--- a/components/sync/protocol/user_event_specifics.proto
+++ b/components/sync/protocol/user_event_specifics.proto
@@ -79,9 +79,15 @@ message UserEventSpecifics {
   // 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;
   }
 }