Add missing IWYU message_loop.h includes.

chrome/browser/prefs/profile_pref_store_manager_unittest.cc

 // Copyright 2014 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 "chrome/browser/prefs/profile_pref_store_manager.h"
 
 #include <stddef.h>
 
 #include <memory>
 #include <utility>
 #include <vector>
 
 #include "base/callback_helpers.h"
 #include "base/compiler_specific.h"
 #include "base/files/file_enumerator.h"
 #include "base/files/file_util.h"
 #include "base/files/scoped_temp_dir.h"
 #include "base/macros.h"
 #include "base/memory/ptr_util.h"
 #include "base/memory/ref_counted.h"
+#include "base/message_loop/message_loop.h"
 #include "base/run_loop.h"
 #include "base/strings/string_util.h"
 #include "base/test/scoped_feature_list.h"
 #include "base/test/sequenced_worker_pool_owner.h"
 #include "base/threading/sequenced_worker_pool.h"
 #include "base/values.h"
 #include "chrome/common/chrome_features.h"
 #include "components/pref_registry/pref_registry_syncable.h"
 #include "components/prefs/json_pref_store.h"
 #include "components/prefs/persistent_pref_store.h"
@@ skipping 597 matching lines @@
   LoadExistingPrefs();
   ExpectStringValueEquals(kProtectedAtomic, kGoodbyeWorld);
   VerifyResetRecorded(false);
 }
 
 // The parameter controls whether the user pref store is created within a
 // service.
 INSTANTIATE_TEST_CASE_P(ProfilePrefStoreManagerTest,
                         ProfilePrefStoreManagerTest,
                         testing::Bool());