Introduce CSSLayoutSetLogger to pass the print result to C# side

Summary:
- CSSLayoutSetLogger
  - Problem: Unity or other logging system can't use printf output
  - Solution: Add CSSLogger to pass CSSNodePrint result to UnityEngine.Debug.Log or other logging system via CSSLogger function for debugging purpose

Reviewed By: emilsjolander

Differential Revision: D4027044

fbshipit-source-id: 90e2e449260888770f71fa7ea790ca9764d91c44

csharp/Facebook.CSSLayout/CSSAssert.cs

@@ -16,14 +16,16 @@ namespace Facebook.CSSLayout
         public delegate void FailFunc(string message);
 
         private static bool _assertInitialized;
+        private static FailFunc _failFunc;
 
         public static void Initialize()
         {
             if (!_assertInitialized)
             {
-                Native.CSSAssertSetFailFunc((message) => {
+                _failFunc = (message) => {
                     throw new InvalidOperationException(message);
-                });
+                };
+                Native.CSSAssertSetFailFunc(_failFunc);
                 _assertInitialized = true;
             }
         }