Add initial Controller to DownloadService

components/download/internal/test/BUILD.gn

 # 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 = [
+    "empty_client.cc",
+    "empty_client.h",
     "entry_utils.cc",
     "entry_utils.h",
+    "mock_client.cc",
+    "mock_client.h",
     "mock_model_client.cc",
     "mock_model_client.h",
     "noop_store.cc",
     "noop_store.h",
     "test_download_driver.cc",
     "test_download_driver.h",
     "test_store.cc",
     "test_store.h",
   ]
 
   public_deps = [
     "//base",
     "//components/download/internal",
     "//components/download/public",
     "//testing/gmock",
   ]
 }