Add CesiumMath.toRadians to externs

Author: gberaudo

Cesium.externs.js

@@ -1848,6 +1848,12 @@ Cesium.Math.TWO_PI;
  */
 Cesium.Math.toDegrees = function(rad) {};
 
+/**
+ * @param {number} degrees
+ * @return {number} angle in radians
+ */
+Cesium.Math.toRadians = function(degrees) {};
+
 /**
  * @constructor
  */