Add missing IWYU message_loop.h includes.

components/data_reduction_proxy/core/browser/data_reduction_proxy_config_unittest.cc

 // Copyright 2015 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/data_reduction_proxy/core/browser/data_reduction_proxy_config.h"
 
 #include <stddef.h>
 #include <stdint.h>
 
 #include <cstdlib>
 #include <map>
 #include <memory>
 #include <utility>
 #include <vector>
 
 #include "base/command_line.h"
 #include "base/files/file_path.h"
 #include "base/files/file_util.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/metrics/field_trial.h"
 #include "base/run_loop.h"
 #include "base/strings/safe_sprintf.h"
 #include "base/strings/string_number_conversions.h"
 #include "base/strings/string_util.h"
 #include "base/test/histogram_tester.h"
 #include "base/test/simple_test_tick_clock.h"
 #include "base/threading/platform_thread.h"
 #include "base/time/time.h"
 #include "components/data_reduction_proxy/core/browser/data_reduction_proxy_config_test_utils.h"
@@ skipping 1355 matching lines @@
   // Sleep to ensure that the delayed task is posted.
   base::PlatformThread::Sleep(base::TimeDelta::FromSeconds(1));
   RunUntilIdle();
   histogram_tester.ExpectTotalCount(
       "DataReductionProxy.LoFi.Accuracy.1.Unknown", 1);
   histogram_tester.ExpectBucketCount(
       "DataReductionProxy.LoFi.Accuracy.1.Unknown", 0, 1);
 }
 
 }  // namespace data_reduction_proxy