Silicon Valley firms have long been known for offering a litany of employee perks: Home-cooked lunches, free massages, climbing walls and dog-friendly offices. Now some are adding yet another incentive to attract — and retain — workers: Paid time off to protest. Fauna, a San Francisco-based database start-up, recently began allowing its 13 employees to take unlimited paid leave to participate in rallies, vote, write letters to elected officials and take part in other civic activities.