Add missing IWYU message_loop.h includes.

components/sync/model/attachments/attachment_service_proxy_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/sync/model/attachments/attachment_service_proxy.h"
 
 #include <memory>
 
 #include "base/bind.h"
 #include "base/location.h"
 #include "base/memory/ptr_util.h"
 #include "base/memory/ref_counted_memory.h"
+#include "base/message_loop/message_loop.h"
 #include "base/run_loop.h"
 #include "base/single_thread_task_runner.h"
 #include "base/synchronization/lock.h"
 #include "base/synchronization/waitable_event.h"
 #include "base/threading/non_thread_safe.h"
 #include "base/threading/thread.h"
 #include "base/threading/thread_task_runner_handle.h"
 #include "components/sync/base/model_type.h"
 #include "components/sync/protocol/sync.pb.h"
 #include "testing/gtest/include/gtest/gtest.h"
@@ skipping 153 matching lines @@
   // don't crash and the count remains the same.
   proxy_->GetOrDownloadAttachments(AttachmentIdList(),
                                    callback_get_or_download_);
   WaitForStubThread();
   WaitForStubThread();
   base::RunLoop().RunUntilIdle();
   EXPECT_EQ(1, count_callback_get_or_download_);
 }
 
 }  // namespace syncer