Fix build and clean up

Summary:
- bit operation with long
- Clean up _measureOutput which is no longer needed
- Fix unittests (SetMeasureFunction, unused error)

Reviewed By: emilsjolander

Differential Revision: D4082401

fbshipit-source-id: b3b2dd002d108c5b43f36a4a73ce8e5233281b45

csharp/Facebook.CSSLayout/MeasureOutput.cs

@@ -13,7 +13,7 @@ namespace Facebook.CSSLayout
     {
         public static long Make(int width, int height)
         {
-          return ((long) width) << 32 | ((long) height);
+          return (long)(((ulong) width) << 32 | ((ulong) height));
         }
 
         public static int GetWidth(long measureOutput)