The original statement is true by the definition of what constitutes a right angle. It's simply set up this way.
The contrapositive would be the statement "if the angle does not measure 90 degrees, then the angle is not a right angle" which is also a true statement. Example: a 37 degree angle is not a right angle.
Note: the original conditional "If P, then Q" would have the contrapositive be in the form "If not Q, then not P". We flip P and Q, and stick "not"s in front of both parts.
