Blac Chyna -- Rob Went In-N-Out ... Now I'm Goin' Burger King (VIDEO)

Blac Chyna has broken ranks in the burger department, making a Burger King run hours after TMZ broke the story ... she is pregnant with Rob Kardashian's kid. Chyna was in downtown L.A. with a friend, scoring a few bags at the drive-thru. Rob -- a…