YGNode: one byte of private storage

Summary:
Adds one byte of private storage to `YGNode`, intended to be used by Yoga itself.

This is in previously unused alignment space, and won’t cause more memory to be allocated.

Reviewed By: SidharthGuglani

Differential Revision: D15296732

fbshipit-source-id: 3caf0a3cd506e4e324e51c31869c69be5781d476

yoga/YGNode.h

@@ -5,6 +5,7 @@
  * file in the root directory of this source tree.
  */
 #pragma once
+#include <cstdint>
 #include <stdio.h>
 #include "CompactValue.h"
 #include "YGConfig.h"
@@ -27,6 +28,7 @@ private:
   bool measureUsesContext_ : 1;
   bool baselineUsesContext_ : 1;
   bool printUsesContext_ : 1;
+  uint8_t reserved_ = 0;
   union {
     YGMeasureFunc noContext;
     MeasureWithContextFn withContext;
@@ -91,6 +93,9 @@ public:
   // Getters
   void* getContext() const { return context_; }
 
+  uint8_t& reserved() { return reserved_; }
+  uint8_t reserved() const { return reserved_; }
+
   void print(void*);
 
   bool getHasNewLayout() const { return hasNewLayout_; }