Make client tag IDs consistent in sync fake server

sync/test/fake_server/unique_client_entity.cc

Index: sync/test/fake_server/unique_client_entity.cc
diff --git a/sync/test/fake_server/unique_client_entity.cc b/sync/test/fake_server/unique_client_entity.cc
index 39d3a4c23d04069c4672d3ca937db02b6fe31332..66f0d4f3d47e880f3b83e36ee4199507558647ec 100644
--- a/sync/test/fake_server/unique_client_entity.cc
+++ b/sync/test/fake_server/unique_client_entity.cc
@@ -28,9 +28,7 @@ FakeServerEntity* UniqueClientEntity::CreateNew(
       << "A UniqueClientEntity must have a client-defined unique tag.";
   ModelType model_type =
       syncer::GetModelTypeFromSpecifics(client_entity.specifics());
-  string id = client_entity.version() == 0 ?
-      FakeServerEntity::CreateId(model_type, base::GenerateGUID()) :
-      client_entity.id_string();
+  string id = EffectiveIdForClientTaggedEntity(client_entity);
   return new UniqueClientEntity(id,
                                 model_type,
                                 client_entity.version(),
@@ -45,7 +43,8 @@ FakeServerEntity* UniqueClientEntity::CreateNew(
 FakeServerEntity* UniqueClientEntity::CreateUpdatedVersion(
     const sync_pb::SyncEntity& client_entity,
     FakeServerEntity* current_server_entity) {
-  return new UniqueClientEntity(client_entity.id_string(),
+  string id = EffectiveIdForClientTaggedEntity(client_entity);
+  return new UniqueClientEntity(id,
                                 current_server_entity->GetModelType(),
                                 client_entity.version(),
                                 client_entity.name(),
@@ -55,6 +54,14 @@ FakeServerEntity* UniqueClientEntity::CreateUpdatedVersion(
                                 client_entity.mtime());
 }
 
+// static
+std::string UniqueClientEntity::EffectiveIdForClientTaggedEntity(
+    const sync_pb::SyncEntity& entity) {
+  return FakeServerEntity::CreateId(
+      syncer::GetModelTypeFromSpecifics(entity.specifics()),
+      entity.client_defined_unique_tag());
+}
+
 UniqueClientEntity::UniqueClientEntity(
     const string& id,
     const ModelType& model_type,