Both Grab and Gojek (which is Indonesia-based and better than Grab IMO) provide helmets. In case your moto driver turns up without one (maybe they forgot/lost/damaged it), you have the right to cancel and pay nothing and book again (which is virtually no inconvenience since wait times are rarely more than single digit minutes unless you're in the middle of nowhere).

Both providers also offer cars which, unless you need a tour guide, is generally cheaper and more convenient than pre-arranged drivers.