Add migration from old to new NTP Promo pref structure.

ios/chrome/browser/notification_promo.h

Index: ios/chrome/browser/notification_promo.h
diff --git a/ios/chrome/browser/notification_promo.h b/ios/chrome/browser/notification_promo.h
index c1441214498557d2c4013f227b3bf1fc49f4322d..d05a69987f0e2c031f96a91abf27350d1416ee13 100644
--- a/ios/chrome/browser/notification_promo.h
+++ b/ios/chrome/browser/notification_promo.h
@@ -87,9 +87,12 @@ class NotificationPromo {
   // 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;
@@ -102,6 +105,12 @@ class NotificationPromo {
   // 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_;