[Sync] Improve MetadataChangeList usage for types using ModelTypeStore.

components/sync/model_impl/in_memory_metadata_change_list.h

Index: components/sync/model_impl/in_memory_metadata_change_list.h
diff --git a/components/sync/model_impl/in_memory_metadata_change_list.h b/components/sync/model_impl/in_memory_metadata_change_list.h
new file mode 100644
index 0000000000000000000000000000000000000000..f077087db83c3f7ee44589da8a84f0778197394d
--- /dev/null
+++ b/components/sync/model_impl/in_memory_metadata_change_list.h
@@ -0,0 +1,55 @@
+// 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_IN_MEMORY_METADATA_CHANGE_LIST_H_
+#define COMPONENTS_SYNC_MODEL_IMPL_IN_MEMORY_METADATA_CHANGE_LIST_H_
+
+#include <map>
+#include <memory>
+#include <string>
+
+#include "components/sync/engine/non_blocking_sync_common.h"
+#include "components/sync/model/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 base class that stores changes in member fields.
+// There are no accessors; subclasses can decide how to expose the changes.
+class InMemoryMetadataChangeList : public MetadataChangeList {
+ public:
+  enum ChangeType { UPDATE, CLEAR };
+
+  struct MetadataChange {
+    ChangeType type;
+    sync_pb::EntityMetadata metadata;
+  };
+
+  struct ModelTypeStateChange {
+    ChangeType type;
+    sync_pb::ModelTypeState state;
+  };
+
+  ~InMemoryMetadataChangeList() override;
+
+  // MetadataChangeList implementation.
+  void UpdateModelTypeState(
+      const sync_pb::ModelTypeState& model_type_state) override;
+  void ClearModelTypeState() override;
+  void UpdateMetadata(const std::string& storage_key,
+                      const sync_pb::EntityMetadata& metadata) override;
+  void ClearMetadata(const std::string& storage_key) override;
+
+ protected:
+  // Protected because this class is useless to instantiate directly.
+  InMemoryMetadataChangeList();
+
+  std::map<std::string, MetadataChange> metadata_changes_;
+  std::unique_ptr<ModelTypeStateChange> state_change_;
+};
+
+}  // namespace syncer
+
+#endif  // COMPONENTS_SYNC_MODEL_IMPL_IN_MEMORY_METADATA_CHANGE_LIST_H_