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/CSSLayout/CSSInterop.h

@@ -0,0 +1,20 @@
+/**
+ * Copyright (c) 2014-present, Facebook, Inc.
+ * All rights reserved.
+ *
+ * This source code is licensed under the BSD-style license found in the
+ * LICENSE file in the root directory of this source tree. An additional grant
+ * of patent rights can be found in the PATENTS file in the same directory.
+ */
+
+#pragma once
+
+#include <CSSLayout/CSSLayout.h>
+
+CSS_EXTERN_C_BEGIN
+
+typedef void (*CSSInteropLoggerFunc)(const char *message);
+
+WIN_EXPORT void CSSInteropSetLogger(CSSInteropLoggerFunc managedFunc);
+
+CSS_EXTERN_C_END