C# use YogaSize throughout the measure call chain

Summary:
Java still has the same problem (see #296)

Also there is no need to have double in the method signature as we only use float internally, which could confuse some.
Closes https://github.com/facebook/yoga/pull/300

Reviewed By: splhack

Differential Revision: D4365993

Pulled By: emilsjolander

fbshipit-source-id: 681f8b1725e63eddcfb9a6c756f2ae215a44425a

csharp/Facebook.Yoga/YogaNode.cs

@@ -532,8 +532,7 @@ namespace Facebook.Yoga
                 throw new InvalidOperationException("Measure function is not defined.");
             }
 
-            long output = _measureFunction(this, width, widthMode, height, heightMode);
-            return new YogaSize { width = MeasureOutput.GetWidth(output), height = MeasureOutput.GetHeight(output) };
+            return _measureFunction(this, width, widthMode, height, heightMode);
         }
 
         public string Print(YogaPrintOptions options =