Use MetadataChangeList and EntityChangeList in SharedModelTypeProcessor

sync/api/model_type_service.h

 // Copyright 2015 The Chromium Authors. All rights reserved.
 // Use of this source code is governed by a BSD-style license that can be
 // found in the LICENSE file.
 
 #ifndef SYNC_API_MODEL_TYPE_SERVICE_H_
 #define SYNC_API_MODEL_TYPE_SERVICE_H_
 
 #include <string>
 #include <vector>
 
@@ skipping 30 matching lines @@
   // model type store.
   virtual scoped_ptr<MetadataChangeList> CreateMetadataChangeList() = 0;
 
   // Perform the initial merge of data from the sync server. Should only need
   // to be called when sync is first turned on, not on every restart.
   virtual syncer::SyncError MergeSyncData(
       scoped_ptr<MetadataChangeList> metadata_change_list,
       EntityDataList entity_data_list) = 0;
 
   // Apply changes from the sync server locally.
+  // Please note that |entity_changes| might have fewer entries than
+  // |metadata_change_list| in case when some of the data changes are filtered
+  // out, or even be empty in case when a commit confirmation is processed and
+  // only the metadata needs to persisted.
   virtual syncer::SyncError ApplySyncChanges(
       scoped_ptr<MetadataChangeList> metadata_change_list,
       EntityChangeList entity_changes) = 0;
 
   // Asynchronously retrieve the sync metadata.
   virtual void LoadMetadata(MetadataCallback callback) = 0;
 
   // Asynchronously retrieve the corresponding sync data for |client_keys|.
   virtual void GetData(ClientKeyList client_keys, DataCallback callback) = 0;
 
@@ skipping 12 matching lines @@
   void clear_change_processor();
 
  private:
   // Recieves ownership in set_change_processor(...).
   scoped_ptr<syncer_v2::ModelTypeChangeProcessor> change_processor_;
 };
 
 }  // namespace syncer_v2
 
 #endif  // SYNC_API_MODEL_TYPE_SERVICE_H_