Add missing IWYU message_loop.h includes.

components/policy/core/common/policy_service_impl_unittest.cc

 // Copyright (c) 2012 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/policy_service_impl.h"
 
 #include <memory>
 #include <utility>
 
 #include "base/bind.h"
 #include "base/bind_helpers.h"
 #include "base/callback.h"
 #include "base/macros.h"
 #include "base/memory/ptr_util.h"
+#include "base/message_loop/message_loop.h"
 #include "base/run_loop.h"
 #include "base/values.h"
 #include "components/policy/core/common/external_data_fetcher.h"
 #include "components/policy/core/common/mock_configuration_policy_provider.h"
 #include "components/policy/core/common/mock_policy_service.h"
 #include "components/policy/core/common/policy_types.h"
 #include "components/policy/policy_constants.h"
 #include "testing/gmock/include/gmock/gmock.h"
 #include "testing/gtest/include/gtest/gtest.h"
 
@@ skipping 672 matching lines @@
                          base::MakeUnique<base::Value>(3), nullptr);
 
   provider0_.UpdatePolicy(std::move(policy_bundle));
   RunUntilIdle();
 
   EXPECT_TRUE(VerifyPolicies(chrome_namespace, expected_chrome));
   EXPECT_TRUE(VerifyPolicies(extension_namespace, expected_extension));
 }
 
 }  // namespace policy