Add migration from old to new NTP Promo pref structure.

ios/chrome/browser/notification_promo.h

 // Copyright (c) 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 IOS_CHROME_BROWSER_NOTIFICATION_PROMO_H_
 #define IOS_CHROME_BROWSER_NOTIFICATION_PROMO_H_
 
 #include <memory>
 #include <string>
 
@@ skipping 69 matching lines @@
 
   // Register preferences.
   static void RegisterPrefs(PrefRegistrySimple* registry);
   static void RegisterProfilePrefs(user_prefs::PrefRegistrySyncable* registry);
   static void MigrateUserPrefs(PrefService* user_prefs);
 
  private:
   // For testing.
   friend class NotificationPromoTest;
 
-  // Flush data members to prefs for storage.
+  // Flush data from instance variables to prefs for storage.
   void WritePrefs();
 
+  // Flush given parameters to prefs for storage.
+  void WritePrefs(int promo_id, double first_view_time, int views, bool closed);
+
   // Tests views_ against max_views_.
   // When max_views_ is 0, we don't cap the number of views.
   bool ExceedsMaxViews() const;
 
   // Tests |first_view_time_| + |max_seconds_| and -now().
   // When either is 0, we don't cap the number of seconds.
   bool ExceedsMaxSeconds() const;
 
   // Returns whether the parameter associated with |param_name| is inside the
   // payload.
   bool IsPayloadParam(const std::string& param_name) const;
 
+  // Transition data saved in old prefs structure to new structure that supports
+  // storing multiple promos.
+  // TODO(crbug.com/623726) Remove this method when migration is no longer
+  // needed as most users have been upgraded to the new pref structure.
+  void MigrateOldPrefs();
+
   PrefService* local_state_;
 
   PromoType promo_type_;
   std::string promo_text_;
 
   std::unique_ptr<const base::DictionaryValue> promo_payload_;
 
   double start_;
   double end_;
   int promo_id_;
@@ skipping 10 matching lines @@
 
   int views_;
   bool closed_;
 
   DISALLOW_COPY_AND_ASSIGN(NotificationPromo);
 };
 
 }  // namespace ios
 
 #endif  // IOS_CHROME_BROWSER_NOTIFICATION_PROMO_H_