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..1148b1b9cbda3fe462561aa73ba543bbd0ae7532 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,
@@ -207,6 +215,10 @@ syncer::SyncMergeResult PrefModelAssociator::MergeDataAndStartSyncing(
     InitPrefAndAssociate(syncer::SyncData(), *pref_name_iter, &new_changes);
   }
 
+  for (const auto& callback : callback_list_)
+    callback.Run();
+  callback_list_.clear();

[pavely, 2016/07/06 22:11:56]

I think it is possible that MergeDataAndStartSyncing is called multiple times
for a given object. Scenario would be:
- Sign-in, enable syncing everything (First time MDaSS is called)
- Disable syncing settings (StopSyncing is called)
- Enable syncing settings (Second time MDaSS is called)

[lshang, 2016/07/07 08:26:05]

I think in this case even though the migration code will be called twice, user's
settings will not change again on the second time, because all the domain scoped
content settings have already been migrated to origin scoped styles and it will
output the same result no matter how many times it be called later on. 

We will also register a pref to control this in the following CL, to make sure
that callback in sync only gets called once.

+
   // Push updates to sync.
   merge_result.set_error(
       sync_processor_->ProcessSyncChanges(FROM_HERE, new_changes));