Add missing IWYU message_loop.h includes.

chrome/browser/prefs/synced_pref_change_registrar_browsertest.cc

 // Copyright 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 <memory>
 #include <string>
 
 #include "base/json/json_string_value_serializer.h"
 #include "base/memory/ptr_util.h"
+#include "base/message_loop/message_loop.h"
 #include "base/time/time.h"
 #include "base/values.h"
 #include "chrome/browser/prefs/pref_service_syncable_util.h"
 #include "chrome/browser/ui/browser.h"
 #include "chrome/common/pref_names.h"
 #include "chrome/test/base/in_process_browser_test.h"
 #include "chrome/test/base/testing_profile.h"
 #include "components/policy/core/browser/browser_policy_connector.h"
 #include "components/policy/core/common/mock_configuration_policy_provider.h"
 #include "components/policy/core/common/policy_map.h"
@@ skipping 180 matching lines @@
   policies.Set(policy::key::kShowHomeButton, policy::POLICY_LEVEL_MANDATORY,
                policy::POLICY_SCOPE_USER, policy::POLICY_SOURCE_CLOUD,
                base::MakeUnique<base::Value>(true), nullptr);
   UpdateChromePolicy(policies);
 
   EXPECT_TRUE(prefs()->IsManagedPreference(prefs::kShowHomeButton));
   SetBooleanPrefValueFromSync(prefs::kShowHomeButton, false);
   EXPECT_FALSE(observer.has_been_notified);
   EXPECT_TRUE(GetBooleanPrefValue(prefs::kShowHomeButton));
 }