Add callback list to PrefModelAssociator after sync data is loaded

components/syncable_prefs/pref_service_syncable.h

Index: components/syncable_prefs/pref_service_syncable.h
diff --git a/components/syncable_prefs/pref_service_syncable.h b/components/syncable_prefs/pref_service_syncable.h
index 2aa7d7f61e51f43b144d668be78d5483b6a9de95..9e467b4bff876358cb27c51bf7db575de61c3b3a 100644
--- a/components/syncable_prefs/pref_service_syncable.h
+++ b/components/syncable_prefs/pref_service_syncable.h
@@ -73,6 +73,8 @@ class PrefServiceSyncable : public PrefService {
   void AddObserver(PrefServiceSyncableObserver* observer);
   void RemoveObserver(PrefServiceSyncableObserver* observer);
 
+  void RegisterMergeDataFinishedCallback(const base::Closure& callback);
+
   // TODO(zea): Have PrefServiceSyncable implement
   // syncer::SyncableService directly.
   syncer::SyncableService* GetSyncableService(const syncer::ModelType& type);