Add YGConfigGetInstanceCount

Summary:
- depends on #496
- For memory leak unit test
- Expose the API for C#
Closes https://github.com/facebook/yoga/pull/497

Reviewed By: emilsjolander

Differential Revision: D4796190

Pulled By: splhack

fbshipit-source-id: 99e4e78e8dfb3d459cf6cd7103ab252c3748e5a6

csharp/Facebook.Yoga/YogaConfig.cs

@@ -63,5 +63,10 @@ namespace Facebook.Yoga
                 Native.YGConfigSetPointScaleFactor(_ygConfig, value);
             }
         }
+
+        public static int GetInstanceCount()
+        {
+            return Native.YGConfigGetInstanceCount();
+        }
     }
 }