Add missing IWYU message_loop.h includes.

components/sync/engine/attachments/attachment_store_test_template.h

 // 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.
 
 #ifndef COMPONENTS_SYNC_ENGINE_ATTACHMENTS_ATTACHMENT_STORE_TEST_TEMPLATE_H_
 #define COMPONENTS_SYNC_ENGINE_ATTACHMENTS_ATTACHMENT_STORE_TEST_TEMPLATE_H_
 
 #include <memory>
 #include <utility>
 
 #include "base/bind.h"
 #include "base/files/scoped_temp_dir.h"
 #include "base/memory/ref_counted_memory.h"
+#include "base/message_loop/message_loop.h"
 #include "base/run_loop.h"
 #include "base/threading/thread_task_runner_handle.h"
 #include "components/sync/engine/attachments/attachment_util.h"
 #include "components/sync/model/attachments/attachment.h"
 #include "components/sync/model/attachments/attachment_store.h"
 #include "components/sync/protocol/sync.pb.h"
 #include "testing/gmock/include/gmock/gmock-matchers.h"
 #include "testing/gtest/include/gtest/gtest.h"
 
 // AttachmentStoreTest defines tests for AttachmentStore. To instantiate these
@@ skipping 489 matching lines @@
                            Read_OneNotFound,
                            Drop_DropTwoButOnlyOneExists,
                            Drop_DoesNotExist,
                            ReadMetadataById,
                            ReadMetadata,
                            SetSyncReference_DropSyncReference);
 
 }  // namespace syncer
 
 #endif  // COMPONENTS_SYNC_ENGINE_ATTACHMENTS_ATTACHMENT_STORE_TEST_TEMPLATE_H_