Implement ping_freshness for update_client.

components/update_client/persisted_data.h

Index: components/update_client/persisted_data.h
diff --git a/components/update_client/persisted_data.h b/components/update_client/persisted_data.h
index d4673b4f373a339cac41ae8c63d3440e20b5af0d..16ec968934975b8d04fca91b2e897ba24ef7f981 100644
--- a/components/update_client/persisted_data.h
+++ b/components/update_client/persisted_data.h
@@ -38,10 +38,15 @@ class PersistedData {
   // -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.
+  // has been fully initialized are ignored. Also sets the PingFreshness.
   void SetDateLastRollCall(const std::vector<std::string>& ids,
                            int datenum) const;

[Sorin Jianu, 2016/04/15 20:26:53]

The const modifier on a setter is debatable, as a general idea:
https://isocpp.org/wiki/faq/const-correctness#logical-vs-physical-state

It is a small issue for this CL and our project but might be worth a thought.

[waffles, 2016/04/15 21:54:20]

Done.