[Sync] DeviceInfoService should format ClientTag in backwards compatible way

components/sync_driver/device_info_service.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_DRIVER_DEVICE_INFO_SERVICE_H_
 #define COMPONENTS_SYNC_DRIVER_DEVICE_INFO_SERVICE_H_
 
 #include <stdint.h>
 
 #include <map>
@@ skipping 58 matching lines @@
   scoped_ptr<sync_driver::DeviceInfo> GetDeviceInfo(
       const std::string& client_id) const override;
   ScopedVector<sync_driver::DeviceInfo> GetAllDeviceInfo() const override;
   void AddObserver(Observer* observer) override;
   void RemoveObserver(Observer* observer) override;
   int CountActiveDevices() const override;
 
  private:
   friend class DeviceInfoServiceTest;
 
+  // Formats ClientTag form DeviceInfoSpecifics.
+  static std::string SpecificsToTag(
+      const sync_pb::DeviceInfoSpecifics& specifics);
+
+  // Extracts cache_guid from ClientTag.
+  static std::string TagToCacheGuid(const std::string& tag);
+
   static scoped_ptr<sync_pb::DeviceInfoSpecifics> CopyToSpecifics(
       const sync_driver::DeviceInfo& info);
 
   // Allocate new DeviceInfo from SyncData.
   static scoped_ptr<sync_driver::DeviceInfo> CopyToModel(
       const sync_pb::DeviceInfoSpecifics& specifics);
   // Conversion as we prepare to hand data to the processor.
   static scoped_ptr<syncer_v2::EntityData> CopyToEntityData(
       const sync_pb::DeviceInfoSpecifics& specifics);
 
@@ skipping 64 matching lines @@
 
   // Should always be last member.
   base::WeakPtrFactory<DeviceInfoService> weak_factory_;
 
   DISALLOW_COPY_AND_ASSIGN(DeviceInfoService);
 };
 
 }  // namespace sync_driver_v2
 
 #endif  // COMPONENTS_SYNC_DRIVER_DEVICE_INFO_SERVICE_H_