Add callback list to PrefModelAssociator after sync data is loaded

components/syncable_prefs/pref_model_associator.cc

Index: components/syncable_prefs/pref_model_associator.cc
diff --git a/components/syncable_prefs/pref_model_associator.cc b/components/syncable_prefs/pref_model_associator.cc
index 4cbdd558881022f6e5abb32c4cd5e53da6e8baf8..dd9d71494251ac0ed3fc456a9a1fd2f213a98f31 100644
--- a/components/syncable_prefs/pref_model_associator.cc
+++ b/components/syncable_prefs/pref_model_associator.cc
@@ -157,6 +157,14 @@ void PrefModelAssociator::InitPrefAndAssociate(
   // we'll send the new user controlled value to the syncer.
 }
 
+void PrefModelAssociator::RegisterMergeDataFinishedCallback(
+    const base::Closure& callback) {
+  if (!models_associated_)
+    callback_list_.push_back(callback);
+  else
+    callback.Run();
+}
+
 syncer::SyncMergeResult PrefModelAssociator::MergeDataAndStartSyncing(
     syncer::ModelType type,
     const syncer::SyncDataList& initial_sync_data,
@@ -213,6 +221,10 @@ syncer::SyncMergeResult PrefModelAssociator::MergeDataAndStartSyncing(
   if (merge_result.error().IsSet())
     return merge_result;
 
+  for (const auto& callback : callback_list_)
+    callback.Run();
+  callback_list_.clear();
+
   models_associated_ = true;
   pref_service_->OnIsSyncingChanged();
   return merge_result;