Add Cesium.Math.negativePiToPi extern.

Author: gberaudo

Cesium.externs.js

@@ -1834,6 +1834,13 @@ Cesium.Math = function() {};
 Cesium.Math.convertLongitudeRange = function(angle) {};
 
 
+/**
+ * @param {number} angle Angle in radians.
+ * @return {number} in the range [-Pi, Pi].
+ */
+Cesium.Math.negativePiToPi = function(angle) {};
+
+
 /**
  * @param {number} value
  * @param {number} min