sync: use progress markers instead of timestamps during GetUpdates

chrome/browser/sync/syncable/model_type.h

Index: chrome/browser/sync/syncable/model_type.h
diff --git a/chrome/browser/sync/syncable/model_type.h b/chrome/browser/sync/syncable/model_type.h
index 2760fbea4f8c25bf0ee8f3b3d49d1bef0360f98c..308f51677463013ed9c67f28e5544a69a055b751 100644
--- a/chrome/browser/sync/syncable/model_type.h
+++ b/chrome/browser/sync/syncable/model_type.h
@@ -93,6 +93,14 @@ ModelType GetModelType(const sync_pb::SyncEntity& sync_entity);
 // prefer using GetModelType where possible.
 ModelType GetModelTypeFromSpecifics(const sync_pb::EntitySpecifics& specifics);
 
+// Determine a model type from the field number of its associated
+// EntitySpecifics extension.
+ModelType GetModelTypeFromExtensionFieldNumber(int field_number);
+
+// Return the field number of the EntitySpecifics extension associated with
+// a model type.
+int GetExtensionFieldNumberFromModelType(ModelType model_type);
+
 // Returns a string that represents the name of |model_type|.
 std::string ModelTypeToString(ModelType model_type);