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..9b7e9987f2159ee32ee6d9d8b931b3dcbfe53db1 100644
--- a/components/syncable_prefs/pref_model_associator.cc
+++ b/components/syncable_prefs/pref_model_associator.cc
@@ -157,6 +157,11 @@ void PrefModelAssociator::InitPrefAndAssociate(
   // we'll send the new user controlled value to the syncer.
 }
 
+void PrefModelAssociator::RegisterMergeDataFinishedCallback(

[raymes, 2016/06/29 03:50:01]

Did you look into whether we should use PrefServiceSyncableObserver instead (you
had mentioned this)? What were your thoughts?

[lshang, 2016/06/30 05:03:36]

PrefServiceSyncableObserver will get called in MergeDataAndStartSyncing() and
StopSyncing(), and StopSyncing() will be called in a list of cases:

- User signed out
- Data type disabled by user through settings
- User enabled passphrase encryption on remote device, but hasn't entered it on
local device yet
- Datatype encountered error
- Browser process shutdown

I also added some log to test this, and it didn't output as I expect (maybe for
some reason, I didn't dig too much). I think it doesn't deserve so much extra
concern or handling of cases to reuse PrefServiceSyncableObserver, compared to
just add a new list of callbacks to pref_model_associator and these callbacks
only get called in MergeDataAndStartSyncing().

+    const base::Closure& callback) {
+  callback_list_.push_back(callback);

[raymes, 2016/06/29 03:50:01]

We should make sure that we only add this if we expect MergeDataAndStartSyncing
to be called in the future. I think we can use models_associated_ here? 

if (!models_associated_)
  callback_list_.push_back(callback);
else
  callback.Run();

[lshang, 2016/06/30 05:03:36]

Done.

+}
+
 syncer::SyncMergeResult PrefModelAssociator::MergeDataAndStartSyncing(
     syncer::ModelType type,
     const syncer::SyncDataList& initial_sync_data,
@@ -207,6 +212,10 @@ syncer::SyncMergeResult PrefModelAssociator::MergeDataAndStartSyncing(
     InitPrefAndAssociate(syncer::SyncData(), *pref_name_iter, &new_changes);
   }
 
+  for (const auto& callback : callback_list_)
+    callback.Run();
+  callback_list_.clear();
+
   // Push updates to sync.
   merge_result.set_error(
       sync_processor_->ProcessSyncChanges(FROM_HERE, new_changes));