Fwiw, I've heard anecdotally from profs in my history department that not majoring in history in undergrad is not a deal-breaker for grad programs; some of them even consider it a plus (I know my advisor does). My writing sample was my seminar paper from my Asian Studies class on human rights in Asia, frex. Writing a good thesis in whatever department will be much more important than whatever department you wrote it in from the perspective of the admissions committees.