Another userjs- API fix

Recently the change API was changed so that all non-preference options
had to be prefixed with 'userjs-'.

We don't need to change pagetriage-lastuse since it's an actual
(though hidden) preference.

Change-Id: Id5b258329b8dd517df80bf6c26e09c4f25d28b96
Merge "Another userjs- API fix"