Add a Model to the download component.

components/download/internal/test_support/BUILD.gn

Index: components/download/internal/test_support/BUILD.gn
diff --git a/components/download/internal/test_support/BUILD.gn b/components/download/internal/test_support/BUILD.gn
new file mode 100644
index 0000000000000000000000000000000000000000..3bfc9cb3aeba667223e41b422bd6cb6d85d7bfcb
--- /dev/null
+++ b/components/download/internal/test_support/BUILD.gn
@@ -0,0 +1,21 @@
+# 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.
+
+source_set("test_support") {
+  visibility = [ "//components/download/internal:unit_tests" ]
+
+  testonly = true
+
+  sources = [
+    "mock_model_client.cc",
+    "mock_model_client.h",
+    "mock_store.cc",
+    "mock_store.h",
+  ]
+
+  public_deps = [
+    "//components/download/internal",
+    "//testing/gmock",
+  ]
+}