Used when calculating total locked column width in processColumns Use shrinkwrapping of child columns if no top level width.
function(column) {
        var result = column.width || 0,
            subcols, len, i;

        // <debug>
        if (column.flex) {
            Ext.Error.raise("Columns which are locked do NOT support a flex width. You must set a width on the " + column.text + "column.");
        }
        // </debug>
        if (!result && column.isGroupHeader) {
            subcols = column.items.items;
            len = subcols.length;
            for (i = 0; i < len; i++) {
                result += this.getColumnWidth(subcols[i]);
            }
        }
        return result;
    }