[Sync] Implements the loopback sync server.

components/sync/engine_impl/loopback_server/persistent_tombstone_entity.h

Index: components/sync/engine_impl/loopback_server/persistent_tombstone_entity.h
diff --git a/components/sync/engine_impl/loopback_server/persistent_tombstone_entity.h b/components/sync/engine_impl/loopback_server/persistent_tombstone_entity.h
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index 0000000000000000000000000000000000000000..ea7236d13949176e6fe99888d918c5faad090226
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+// 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_IMPL_LOOPBACK_SERVER_PERSISTENT_TOMBSTONE_ENTITY_H_
+#define COMPONENTS_SYNC_ENGINE_IMPL_LOOPBACK_SERVER_PERSISTENT_TOMBSTONE_ENTITY_H_
+
+#include <memory>
+#include <string>
+
+#include "components/sync/base/model_type.h"
+#include "components/sync/engine_impl/loopback_server/loopback_server_entity.h"
+#include "components/sync/protocol/sync.pb.h"
+
+namespace syncer {
+
+// A Sync entity that represents a deleted item.
+class PersistentTombstoneEntity : public LoopbackServerEntity {
+ public:
+  ~PersistentTombstoneEntity() override;
+
+  // Factory function for PersistentTombstoneEntity.
+  static std::unique_ptr<LoopbackServerEntity> Create(
+      const sync_pb::SyncEntity& id);
+
+  // LoopbackServerEntity implementation.
+  bool RequiresParentId() const override;
+  std::string GetParentId() const override;
+  void SerializeAsProto(sync_pb::SyncEntity* proto) const override;
+  bool IsDeleted() const override;
+  sync_pb::LoopbackServerEntity_Type GetLoopbackServerEntityType()
+      const override;
+
+ private:
+  PersistentTombstoneEntity(const std::string& id,
+                            int64_t version,
+                            const syncer::ModelType& model_type);
+};
+
+}  // namespace syncer
+
+#endif  // COMPONENTS_SYNC_ENGINE_IMPL_LOOPBACK_SERVER_PERSISTENT_TOMBSTONE_ENTITY_H_