Add missing IWYU message_loop.h includes.

components/sync/model_impl/accumulating_metadata_change_list_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/sync/model_impl/accumulating_metadata_change_list.h"
 
 #include <set>
 
 #include "base/bind.h"
+#include "base/message_loop/message_loop.h"
 #include "base/run_loop.h"
 #include "components/sync/model/mock_model_type_store.h"
 #include "testing/gtest/include/gtest/gtest.h"
 
 namespace syncer {
 namespace {
 
 using WriteBatch = ModelTypeStore::WriteBatch;
 
 void RecordMetadataWrite(std::map<std::string, std::string>* map,
@@ skipping 177 matching lines @@
   cl.TransferChanges(store(), batch.get());
 
   EXPECT_TRUE(delete_set.find("client_tag") != delete_set.end());
   EXPECT_TRUE(change_map.find("client_tag") == change_map.end());
   EXPECT_TRUE(delete_called);
   EXPECT_TRUE(global_metadata.empty());
 }
 
 }  // namespace
 }  // namespace syncer