Sync: Don't set/use Parent ID for non-hierarchical sync data types.

sync/engine/syncer_proto_util.h

Index: sync/engine/syncer_proto_util.h
diff --git a/sync/engine/syncer_proto_util.h b/sync/engine/syncer_proto_util.h
index d58189abfb2dfc6d36de938f3929d83cbb6a1b04..2a7bd34793dac2197be2c13e5940a717ec23e166 100644
--- a/sync/engine/syncer_proto_util.h
+++ b/sync/engine/syncer_proto_util.h
@@ -58,8 +58,14 @@ class SYNC_EXPORT_PRIVATE SyncerProtoUtil {
       sessions::SyncSession* session,
       ModelTypeSet* partial_failure_data_types);
 
+  // Specifies where entity's position should be updated from the data in
+  // GetUpdates message.
   static bool ShouldMaintainPosition(const sync_pb::SyncEntity& sync_entity);
 
+  // Specifies where entity's parent ID should be updated from the data in
+  // GetUpdates message.
+  static bool ShouldMaintainHierarchy(const sync_pb::SyncEntity& sync_entity);
+
   // Extract the name field from a sync entity.
   static const std::string& NameFromSyncEntity(
       const sync_pb::SyncEntity& entry);