sync: Process 'control' data separately

sync/internal_api/public/syncable/model_type.h

Index: sync/internal_api/public/syncable/model_type.h
diff --git a/sync/internal_api/public/syncable/model_type.h b/sync/internal_api/public/syncable/model_type.h
index c8a9905dfe478b338c24bdac0dd8fdd143c25de4..f6014022ec6bc419c4b7678a140fb58f73353d66 100644
--- a/sync/internal_api/public/syncable/model_type.h
+++ b/sync/internal_api/public/syncable/model_type.h
@@ -119,6 +119,12 @@ ModelType GetModelTypeFromSpecifics(const sync_pb::EntitySpecifics& specifics);
 // value (sibling ordering) for this item.
 bool ShouldMaintainPosition(ModelType model_type);
 
+// If this returns true, this type belongs in GROUP_CONTROL.  The syncer has
+// special code to deal with its encryption, update application and conflict
+// resolution, so it should not receive any 'generic' processing for those
+// purposes.
+bool IsControlType(ModelType model_type);
+
 // Determine a model type from the field number of its associated
 // EntitySpecifics field.
 ModelType GetModelTypeFromSpecificsFieldNumber(int field_number);