Use a direct include of the message_loop header in base/.

base/metrics/field_trial_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 "base/metrics/field_trial.h"
 
-#include "base/message_loop.h"
+#include "base/message_loop/message_loop.h"
 #include "base/rand_util.h"
 #include "base/run_loop.h"
 #include "base/strings/string_number_conversions.h"
 #include "base/strings/stringprintf.h"
 #include "testing/gtest/include/gtest/gtest.h"
 
 namespace base {
 
 namespace {
 
@@ skipping 866 matching lines @@
 
   FieldTrial* trial =
       FieldTrialList::FactoryGetFieldTrial(
           kTrialName, kProbability, kDefaultGroupName,
           FieldTrialList::kNoExpirationYear, 1, 1, NULL);
   trial->AppendGroup(kGroupName, kProbability);
   EXPECT_EQ(kGroupName, trial->group_name());
 }
 
 }  // namespace base