The normal force for circular motion is from normal component of gravitation force, and it should be larger than the centripetal force
i.e. N= mg cos\theta\ge m \frac{v^2}{R}, where R is the radius.
Since it starts to roll down from the top,
the kinetic energy is coming from changes of potential energy
So \frac{v^2}{R}=2mg(1-cos\theta)\le mg cos\theta
2mg\le3mg cos\theta
The result is \theta\le cos^{-1}\frac{2}{3}