Add a Model to the download component.

components/download/internal/test_support/mock_store.cc

Index: components/download/internal/test_support/mock_store.cc
diff --git a/components/download/internal/test_support/mock_store.cc b/components/download/internal/test_support/mock_store.cc
new file mode 100644
index 0000000000000000000000000000000000000000..0431ae8fd6cc868117a7e401de35b8397a06af5d
--- /dev/null
+++ b/components/download/internal/test_support/mock_store.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_store.h"
+
+namespace download {
+
+MockStore::MockStore() = default;
+MockStore::~MockStore() = default;
+
+}  // namespace download