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.cpp

@@ -0,0 +1,30 @@
+/**
+ * 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.
+ */
+
+#include "CSSInterop.h"
+
+static CSSInteropLoggerFunc gManagedFunc;
+
+static int unmanagedLogger(const char *format, ...) {
+  int result = 0;
+  if (gManagedFunc) {
+    va_list args;
+    va_start(args, format);
+    char buffer[256];
+    result = vsnprintf(buffer, sizeof(buffer), format, args);
+    (*gManagedFunc)(buffer);
+    va_end(args);
+  }
+  return result;
+}
+
+void CSSInteropSetLogger(CSSInteropLoggerFunc managedFunc) {
+  gManagedFunc = managedFunc;
+  CSSLayoutSetLogger(&unmanagedLogger);
+}