Add missing IWYU message_loop.h includes.

components/policy/core/common/cloud/component_cloud_policy_service_unittest.cc

 // Copyright (c) 2013 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/policy/core/common/cloud/component_cloud_policy_service.h"
 
 #include <stdint.h>
 
 #include <map>
 #include <string>
 #include <utility>
 
 #include "base/callback.h"
 #include "base/files/scoped_temp_dir.h"
 #include "base/memory/ptr_util.h"
+#include "base/message_loop/message_loop.h"
 #include "base/run_loop.h"
 #include "base/single_thread_task_runner.h"
 #include "base/stl_util.h"
 #include "base/values.h"
 #include "components/policy/core/common/cloud/cloud_policy_constants.h"
 #include "components/policy/core/common/cloud/mock_cloud_policy_client.h"
 #include "components/policy/core/common/cloud/mock_cloud_policy_store.h"
 #include "components/policy/core/common/cloud/policy_builder.h"
 #include "components/policy/core/common/cloud/resource_cache.h"
 #include "components/policy/core/common/external_data_fetcher.h"
@@ skipping 669 matching lines @@
   RunUntilIdle();
   Mock::VerifyAndClearExpectations(&delegate_);
 
   // The policy is now being served.
   PolicyBundle expected_bundle;
   expected_bundle.Get(kTestExtensionNS).CopyFrom(expected_policy_);
   EXPECT_TRUE(service_->policy().Equals(expected_bundle));
 }
 
 }  // namespace policy