[Sync] Improve MetadataChangeList usage for types using ModelTypeStore.

components/sync/model_impl/accumulating_metadata_change_list.h

Index: components/sync/model_impl/accumulating_metadata_change_list.h
diff --git a/components/sync/model_impl/accumulating_metadata_change_list.h b/components/sync/model_impl/accumulating_metadata_change_list.h
new file mode 100644
index 0000000000000000000000000000000000000000..e810d3a8b2cdc228b3fa0159ac286bf4a67a5edd
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+++ b/components/sync/model_impl/accumulating_metadata_change_list.h
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+// 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.
+
+#ifndef COMPONENTS_SYNC_MODEL_IMPL_ACCUMULATING_METADATA_CHANGE_LIST_H_
+#define COMPONENTS_SYNC_MODEL_IMPL_ACCUMULATING_METADATA_CHANGE_LIST_H_
+
+#include <map>
+#include <memory>
+#include <string>
+
+#include "components/sync/engine/non_blocking_sync_common.h"
+#include "components/sync/model/model_type_store.h"
+#include "components/sync/model_impl/in_memory_metadata_change_list.h"
+#include "components/sync/protocol/entity_metadata.pb.h"
+#include "components/sync/protocol/model_type_state.pb.h"
+
+namespace syncer {
+
+// A MetadataChangeList implementation that is meant to be used in combination
+// with a ModelTypeStore. It accumulates changes in member fields, and then when
+// requested transfers them to the store/write batch.
+class AccumulatingMetadataChangeList : public InMemoryMetadataChangeList {
+ public:
+  AccumulatingMetadataChangeList();
+  ~AccumulatingMetadataChangeList() override;
+
+  // Moves all currently accumulated changes into the write batch, clear out
+  // local copies. Calling this multiple times will work, but should not be
+  // necessary.
+  void TransferChanges(ModelTypeStore* store,
+                       ModelTypeStore::WriteBatch* write_batch);
+};
+
+}  // namespace syncer
+
+#endif  // COMPONENTS_SYNC_MODEL_IMPL_ACCUMULATING_METADATA_CHANGE_LIST_H_