Preliminary implementation for the aggregate sum function as a window function

This implementation does not deal with the case where removal of
elements from the window frame causes the item to turn to a null value.

sql/sql_window.cc

@@ -1584,6 +1584,7 @@ bool JOIN::process_window_functions(List<Item> *curr_fields_list)
             }
           case Item_sum::COUNT_FUNC:
           case Item_sum::SUM_BIT_FUNC:
+          case Item_sum::SUM_FUNC:
           {
             /*
               Frame-aware window function computation. It does one pass, but