Add a Model to the download component.

components/download/internal/test_support/mock_model_client.cc

Index: components/download/internal/test_support/mock_model_client.cc
diff --git a/components/download/internal/test_support/mock_model_client.cc b/components/download/internal/test_support/mock_model_client.cc
new file mode 100644
index 0000000000000000000000000000000000000000..dbe5e1f11c28638eda5ab7d442a303d82a212e82
--- /dev/null
+++ b/components/download/internal/test_support/mock_model_client.cc
@@ -0,0 +1,12 @@
+// 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.
+
+#include "components/download/internal/test_support/mock_model_client.h"
+
+namespace download {
+
+MockModelClient::MockModelClient() = default;
+MockModelClient::~MockModelClient() = default;
+
+}  // namespace download