Allow embedder to set promise internal field count

include/v8.h

Index: include/v8.h
diff --git a/include/v8.h b/include/v8.h
index 099dfb25b434c45a949d4ddf33b2e9c8913881de..5f9cf3e28423720d475bf798303091b7f841ad07 100644
--- a/include/v8.h
+++ b/include/v8.h
@@ -3856,6 +3856,10 @@ class V8_EXPORT Function : public Object {
   static void CheckCast(Value* obj);
 };
 
+#ifndef V8_PROMISE_INTERNAL_FIELD_COUNT
+// The number of required internal fields can be defined by embedder.
+#define V8_PROMISE_INTERNAL_FIELD_COUNT 0
+#endif
 
 /**
  * An instance of the built-in Promise constructor (ES6 draft).
@@ -3937,6 +3941,8 @@ class V8_EXPORT Promise : public Object {
 
   V8_INLINE static Promise* Cast(Value* obj);
 
+  static const int kEmbedderFieldCount = V8_PROMISE_INTERNAL_FIELD_COUNT;
+
  private:
   Promise();
   static void CheckCast(Value* obj);