[USS] Add SyncMetadataStore interface

components/sync/model/sync_metadata_store.h

Index: components/sync/model/sync_metadata_store.h
diff --git a/components/sync/model/sync_metadata_store.h b/components/sync/model/sync_metadata_store.h
new file mode 100644
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+++ b/components/sync/model/sync_metadata_store.h
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+// Copyright 2017 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_SYNC_METADATA_STORE_H_
+#define COMPONENTS_SYNC_MODEL_SYNC_METADATA_STORE_H_
+
+#include <string>
+
+#include "components/sync/base/model_type.h"
+
+namespace sync_pb {
+class EntityMetadata;
+class ModelTypeState;
+}  // namespace sync_pb
+
+namespace syncer {
+
+// SyncMetadataStore defines interface implemented by model types for persisting
+// sync metadata and datatype state. It allows model type to use common
+// implementation of MetadataChangeList (SyncMetadataStoreChangeList) instead of
+// implementing their own.
+// Model type in implementation of ModelTypeSyncBridge::CreateMetadataChangeList
+// should create instance of SyncMetadataStoreChangeList passing pointer to
+// SyncMetadataStore to its constructor.
+// Implementations of SyncMetadataStore methods should support add/update/delete
+// metadata in model type specific sync metadata storage.
+
+class SyncMetadataStore {
+ public:
+  SyncMetadataStore() {}
+  virtual ~SyncMetadataStore() {}
+
+  // Update the metadata row for |model_type|, keyed by |storage_key|, to
+  // contain the contents of |metadata|.
+  // Return true on success.
+  virtual bool UpdateSyncMetadata(syncer::ModelType model_type,
+                                  const std::string& storage_key,
+                                  const sync_pb::EntityMetadata& metadata) = 0;
+
+  // Remove the metadata row of type |model_type| keyed by |storage_key|.
+  // Return true on success.
+  virtual bool ClearSyncMetadata(syncer::ModelType model_type,
+                                 const std::string& storage_key) = 0;
+
+  // Update the stored sync state for the |model_type|.
+  // Return true on success.
+  virtual bool UpdateModelTypeState(
+      syncer::ModelType model_type,
+      const sync_pb::ModelTypeState& model_type_state) = 0;
+
+  // Clear the stored sync state for |model_type|.
+  // Return true on success.
+  virtual bool ClearModelTypeState(syncer::ModelType model_type) = 0;
+};
+
+}  // namespace syncer
+
+#endif  // COMPONENTS_SYNC_MODEL_SYNC_METADATA_STORE_H_