Event participation payments not correctly recorded in financial_trxn table (frontend)

Entries in the civicrm_financial_trxn table are not getting the payment processor id set, leading to an incorrect value in the to_financial_account_id, likely from the payment_instrument_id corresponding setting. I've only seen this happening for transactions from event participation, it's not happening for membership or regular contributions. It appears to be a 4.7 regression, I don't see it happening from when the site used 4.6, although the relevant code seems to have been in there since forever.

This is related to https://issues.civicrm.org/jira/browse/CRM-19213 where it was identified that the payment instrument was wrong, but in that issue it wasn't identified that the payment processor id field in the trxn record was also wrong (though only in the case of registrations, I think the contributions and memberships cases work, at least I haven't seen them as an issue).