Implement ping_freshness for update_client.

components/update_client/persisted_data.h

 // Copyright 2016 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_PERSISTED_DATA_H_
 #define COMPONENTS_UPDATE_CLIENT_PERSISTED_DATA_H_
 
 #include <string>
 #include <vector>
 
@@ skipping 20 matching lines @@
   // The |pref_service| must outlive the entire update_client.
   explicit PersistedData(PrefService* pref_service);
 
   ~PersistedData();
 
   // Returns the DateLastRollCall (the server-localized calendar date number the
   // |id| was last checked by this client on) for the specified |id|.
   // -2 indicates that there is no recorded date number for the |id|.
   int GetDateLastRollCall(const std::string& id) const;
 
+  // Returns the PingFreshness (a random token that is written into the profile
+  // data whenever the DateLastRollCall it is modified) for the specified |id|.
+  // "" indicates that there is no recorded freshness value for the |id|.
+  std::string GetPingFreshness(const std::string& id) const;
+
   // Records the DateLastRollCall for the specified |ids|. |datenum| must be a
   // non-negative integer: calls with a negative |datenum| are simply ignored.
   // Calls to SetDateLastRollCall that occur prior to the persisted data store
-  // has been fully initialized are ignored.
-  void SetDateLastRollCall(const std::vector<std::string>& ids,
-                           int datenum) const;
+  // has been fully initialized are ignored. Also sets the PingFreshness.
+  void SetDateLastRollCall(const std::vector<std::string>& ids, int datenum);
 
   // This is called only via update_client's RegisterUpdateClientPreferences.
   static void RegisterPrefs(PrefRegistrySimple* registry);
 
  private:
   base::ThreadChecker thread_checker_;
   PrefService* pref_service_;
 
   DISALLOW_COPY_AND_ASSIGN(PersistedData);
 };
 
 }  // namespace update_client
 
 #endif  // COMPONENTS_UPDATE_CLIENT_PERSISTED_DATA_H_