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

CSSLayout/CSSLayout.h

@@ -11,6 +11,7 @@
 
 #include <assert.h>
 #include <math.h>
+#include <stdarg.h>
 #include <stdint.h>
 #include <stdio.h>
 #include <stdlib.h>
@@ -121,6 +122,7 @@ typedef CSSSize (*CSSMeasureFunc)(void *context,
                                   float height,
                                   CSSMeasureMode heightMode);
 typedef void (*CSSPrintFunc)(void *context);
+typedef int (*CSSLogger)(const char *format, ...);
 
 #ifdef CSS_ASSERT_FAIL_ENABLED
 typedef void (*CSSAssertFailFunc)(const char *message);
@@ -210,6 +212,8 @@ CSS_NODE_LAYOUT_PROPERTY(float, Width);
 CSS_NODE_LAYOUT_PROPERTY(float, Height);
 CSS_NODE_LAYOUT_PROPERTY(CSSDirection, Direction);
 
+WIN_EXPORT void CSSLayoutSetLogger(CSSLogger logger);
+
 #ifdef CSS_ASSERT_FAIL_ENABLED
 // Assert
 WIN_EXPORT void CSSAssertSetFailFunc(CSSAssertFailFunc func);