Add missing IWYU message_loop.h includes.

components/leveldb_proto/proto_database_impl_unittest.cc

 // Copyright 2014 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/leveldb_proto/proto_database_impl.h"
 
 #include <stddef.h>
 
 #include <map>
 #include <memory>
 #include <utility>
 
 #include "base/bind.h"
 #include "base/files/file_util.h"
 #include "base/files/scoped_temp_dir.h"
 #include "base/location.h"
 #include "base/memory/ptr_util.h"
+#include "base/message_loop/message_loop.h"
 #include "base/run_loop.h"
 #include "base/threading/thread.h"
 #include "base/trace_event/memory_dump_manager.h"
 #include "base/trace_event/process_memory_dump.h"
 #include "components/leveldb_proto/leveldb_database.h"
 #include "components/leveldb_proto/testing/proto/test.pb.h"
 #include "testing/gmock/include/gmock/gmock.h"
 #include "testing/gtest/include/gtest/gtest.h"
 #include "third_party/leveldatabase/src/include/leveldb/options.h"
 
@@ skipping 624 matching lines @@
           ->system_allocator_pool_name();
   size_t expected_dump_count = system_allocator_pool_name ? 2 : 1;
   EXPECT_EQ(expected_dump_count, allocator_dumps.size());
   for (const auto& dump : allocator_dumps) {
     ASSERT_TRUE(dump.first.find("leveldb/leveldb_proto/") == 0 ||
                 dump.first.find(system_allocator_pool_name) == 0);
   }
 }
 
 }  // namespace leveldb_proto