Implement ping_freshness for update_client.

components/update_client/update_checker.h

 // 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_UPDATE_CLIENT_UPDATE_CHECKER_H_
 #define COMPONENTS_UPDATE_CLIENT_UPDATE_CHECKER_H_
 
 #include <string>
 #include <vector>
 
@@ skipping 14 matching lines @@
 
 class UpdateChecker {
  public:
   using UpdateCheckCallback =
       base::Callback<void(int error,
                           const UpdateResponse::Results& results,
                           int retry_after_sec)>;
 
   using Factory =
       scoped_ptr<UpdateChecker> (*)(const scoped_refptr<Configurator>& config,
-                                    const PersistedData& persistent);
+                                    PersistedData* persistent);
 
   virtual ~UpdateChecker() {}
 
   // Initiates an update check for the |items_to_check|. |additional_attributes|
   // provides a way to customize the <request> element. This value is inserted
   // as-is, therefore it must be well-formed as an XML attribute string.
   virtual bool CheckForUpdates(
       const std::vector<CrxUpdateItem*>& items_to_check,
       const std::string& additional_attributes,
       const UpdateCheckCallback& update_check_callback) = 0;
 
   static scoped_ptr<UpdateChecker> Create(
       const scoped_refptr<Configurator>& config,
-      const PersistedData& persistent);
+      PersistedData* persistent);
 
  protected:
   UpdateChecker() {}
 
  private:
   DISALLOW_COPY_AND_ASSIGN(UpdateChecker);
 };
 
 }  // namespace update_client
 
 #endif  // COMPONENTS_UPDATE_CLIENT_UPDATE_CHECKER_H_