Add callback list to PrefModelAssociator after sync data is loaded

components/syncable_prefs/pref_service_syncable.cc

Index: components/syncable_prefs/pref_service_syncable.cc
diff --git a/components/syncable_prefs/pref_service_syncable.cc b/components/syncable_prefs/pref_service_syncable.cc
index c656381877d93ecb1b7b2fe9de01a346014ecac8..41fe2961741b19c9440259a897f24452f3a243be 100644
--- a/components/syncable_prefs/pref_service_syncable.cc
+++ b/components/syncable_prefs/pref_service_syncable.cc
@@ -153,6 +153,11 @@ void PrefServiceSyncable::RemoveSyncedPrefObserver(
   priority_pref_sync_associator_.RemoveSyncedPrefObserver(name, observer);
 }
 
+void PrefServiceSyncable::RegisterMergeDataFinishedCallback(
+    const base::Closure& callback) {
+  pref_sync_associator_.RegisterMergeDataFinishedCallback(callback);
+}
+
 // Set the PrefModelAssociatorClient to use for that object during tests.
 void PrefServiceSyncable::SetPrefModelAssociatorClientForTesting(
     const PrefModelAssociatorClient* pref_model_associator_client) {