[Sync] Convert NonBlockingDataTypeController to be single-thread.

components/sync/driver/fake_sync_client.cc

Index: components/sync/driver/fake_sync_client.cc
diff --git a/components/sync/driver/fake_sync_client.cc b/components/sync/driver/fake_sync_client.cc
index 9e5ee432f0289e124e7f3125d6ad30b5905d4fda..4382cf7d55840c26d12974a112ea4f02028d2684 100644
--- a/components/sync/driver/fake_sync_client.cc
+++ b/components/sync/driver/fake_sync_client.cc
@@ -96,9 +96,9 @@ FakeSyncClient::GetSyncableServiceForType(syncer::ModelType type) {
   return base::WeakPtr<syncer::SyncableService>();
 }
 
-syncer_v2::ModelTypeService* FakeSyncClient::GetModelTypeServiceForType(
-    syncer::ModelType type) {
-  return model_type_service_;
+base::WeakPtr<syncer_v2::ModelTypeService>
+FakeSyncClient::GetModelTypeServiceForType(syncer::ModelType type) {
+  return model_type_service_->AsWeakPtr();
 }
 
 scoped_refptr<syncer::ModelSafeWorker>