Make client tag IDs consistent in sync fake server

sync/test/fake_server/unique_client_entity.h

Index: sync/test/fake_server/unique_client_entity.h
diff --git a/sync/test/fake_server/unique_client_entity.h b/sync/test/fake_server/unique_client_entity.h
index fecf0440412bed680ef4033224798013a0c3fae6..289034e54b85c6cc88ca8a8e5b6a4280f097377f 100644
--- a/sync/test/fake_server/unique_client_entity.h
+++ b/sync/test/fake_server/unique_client_entity.h
@@ -28,6 +28,10 @@ class UniqueClientEntity : public FakeServerEntity {
       const sync_pb::SyncEntity& client_entity,
       FakeServerEntity* current_server_entity);
 
+  // Derives an ID from a unique client tagged entity.
+  static std::string EffectiveIdForClientTaggedEntity(
+      const sync_pb::SyncEntity& entity);
+
   // FakeServerEntity implementation.
   virtual std::string GetParentId() const OVERRIDE;
   virtual sync_pb::SyncEntity* SerializeAsProto() OVERRIDE;