Add getUndefined() method to obtain the undefined value

Summary:
Add getUndefined() method to obtain the undefined value.
This would allow to obtain the Yoga undefined value in runtime, and not just in compile time

Reviewed By: davidaurelio

Differential Revision: D13136972

fbshipit-source-id: aa198aa1ea65bb6b7302abeba6f9f5d483a45ff3

java/com/facebook/yoga/YogaConstants.java

@@ -35,4 +35,8 @@ public class YogaConstants {
   public static boolean isUndefined(YogaValue value) {
     return value.unit == YogaUnit.UNDEFINED;
   }
+
+  public static float getUndefined() {
+    return UNDEFINED;
+  }
 }