Add missing IWYU message_loop.h includes.

components/policy/core/common/policy_loader_mac_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_loader_mac.h"
 
 #include <CoreFoundation/CoreFoundation.h>
 
 #include <utility>
 
 #include "base/callback.h"
 #include "base/files/file_path.h"
 #include "base/mac/scoped_cftyperef.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/strings/sys_string_conversions.h"
 #include "base/values.h"
 #include "components/policy/core/common/async_policy_provider.h"
 #include "components/policy/core/common/configuration_policy_provider_test.h"
 #include "components/policy/core/common/external_data_fetcher.h"
 #include "components/policy/core/common/policy_bundle.h"
 #include "components/policy/core/common/policy_map.h"
 #include "components/policy/core/common/policy_test_utils.h"
 #include "components/policy/core/common/policy_types.h"
@@ skipping 171 matching lines @@
   base::RunLoop().RunUntilIdle();
   PolicyBundle expected_bundle;
   expected_bundle.Get(PolicyNamespace(POLICY_DOMAIN_CHROME, std::string()))
       .Set(test_keys::kKeyString, POLICY_LEVEL_RECOMMENDED, POLICY_SCOPE_USER,
            POLICY_SOURCE_PLATFORM,
            base::MakeUnique<base::Value>("string value"), nullptr);
   EXPECT_TRUE(provider_->policies().Equals(expected_bundle));
 }
 
 }  // namespace policy