How to use a pressure cooker for frozen chicken?
Based on the recipe to which you linked, I'd guess about 10 minutes after the pressure cooker starts steaming. (Until the chicken thaws and starts cooking you shouldn't get much steam.)

Since you're going to shred the chicken and put in a sauce anyway, overcooking might not be a problem. 

Frankly if you have an extra hour put the chicken breast in a plastic bag and fill the pressure cooker with hot water from the tap. Dunk the bag in the pot to thaw the breasts so that you can control the cooking time more closely.