[Sync] Adding storage key concept for ModelTypeServices.

components/sync/api/entity_change.h

 // Copyright 2015 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_API_ENTITY_CHANGE_H_
 #define COMPONENTS_SYNC_API_ENTITY_CHANGE_H_
 
 #include <string>
 #include <vector>
 
 #include "components/sync/api/entity_data.h"
 #include "components/sync/base/sync_export.h"
 
 namespace syncer_v2 {
 
 class SYNC_EXPORT EntityChange {
  public:
   enum ChangeType { ACTION_ADD, ACTION_UPDATE, ACTION_DELETE };
 
-  static EntityChange CreateAdd(std::string client_tag, EntityDataPtr data);
-  static EntityChange CreateUpdate(std::string client_tag, EntityDataPtr data);
-  static EntityChange CreateDelete(std::string client_tag);
+  static EntityChange CreateAdd(const std::string& storage_key,
+                                EntityDataPtr data);
+  static EntityChange CreateUpdate(const std::string& storage_key,
+                                   EntityDataPtr data);
+  static EntityChange CreateDelete(const std::string& storage_key);
 
   EntityChange(const EntityChange& other);
   virtual ~EntityChange();
 
-  std::string client_tag() const { return client_tag_; }
+  std::string storage_key() const { return storage_key_; }
   ChangeType type() const { return type_; }
   const EntityData& data() const { return data_.value(); }
 
  private:
-  EntityChange(std::string client_tag, ChangeType type, EntityDataPtr data);
+  EntityChange(const std::string& storage_key,
+               ChangeType type,
+               EntityDataPtr data);
 
-  std::string client_tag_;
+  std::string storage_key_;
   ChangeType type_;
   EntityDataPtr data_;
 };
 
 typedef std::vector<EntityChange> EntityChangeList;
 
 }  // namespace syncer_v2
 
 #endif  // COMPONENTS_SYNC_API_ENTITY_CHANGE_H_