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/MeasureFunction.cs

@@ -9,7 +9,7 @@
 
 namespace Facebook.Yoga
 {
-    public delegate long MeasureFunction(
+    public delegate YogaSize MeasureFunction(
         YogaNode node,
         float width,
         YogaMeasureMode widthMode,