If done on a horizontal surface, it would be the normal contact force holding up the weight of the bike, and the frictional force providing the required centripetal force.
When done on a vertical surface (as in this video), it is the frictional force holding up the weight of the bike, and the normal contact force providing the required centripetal force.
The key to pulling off this stunt successfully is to ride at a high speed. This forces the bike and wall to be pressed hard into each other, resulting in a large normal contact force. A large normal contact force is crucial because the amount of friction depends on it.
xmDemo 