[IMP] web_editor: put rollback code assignation on its own line

If the `_toRollback` property of the editor is assigned on its own line, it is
easier to identify which assignation caused/prevented a rollback when
debugging.

closes odoo/odoo#130830

X-original-commit: 263dcbaf
Signed-off-by: David Monjoie (dmo) <dmo@odoo.com>