Add missing IWYU message_loop.h includes.

components/ntp_tiles/popular_sites_impl_unittest.cc

 // Copyright 2016 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/ntp_tiles/popular_sites_impl.h"
 
 #include <map>
 #include <string>
 #include <utility>
 #include <vector>
 
 #include "base/bind.h"
 #include "base/command_line.h"
 #include "base/files/file_util.h"
 #include "base/files/important_file_writer.h"
 #include "base/files/scoped_temp_dir.h"
 #include "base/json/json_writer.h"
 #include "base/memory/ptr_util.h"
+#include "base/message_loop/message_loop.h"
 #include "base/optional.h"
 #include "base/run_loop.h"
 #include "base/test/scoped_feature_list.h"
 #include "base/test/sequenced_worker_pool_owner.h"
 #include "base/threading/thread_task_runner_handle.h"
 #include "base/values.h"
 #include "components/ntp_tiles/constants.h"
 #include "components/ntp_tiles/json_unsafe_parser.h"
 #include "components/ntp_tiles/pref_names.h"
 #include "components/ntp_tiles/switches.h"
@@ skipping 437 matching lines @@
 
   PopularSites::SitesVector sites;
   EXPECT_THAT(FetchPopularSites(/*force_download=*/false, &sites),
               Eq(base::Optional<bool>(true)));
 
   EXPECT_THAT(sites.size(), Eq(1u));
 }
 
 }  // namespace
 }  // namespace ntp_tiles