Allow embedder to set promise internal field count

src/objects.h

Index: src/objects.h
diff --git a/src/objects.h b/src/objects.h
index bcb4cd49b7ddf8af12e69723e26f6cc98636fcc3..f94262b7b208ba4c16162c4a0856bfd05cde5ca7 100644
--- a/src/objects.h
+++ b/src/objects.h
@@ -7375,6 +7375,8 @@ class JSPromise : public JSObject {
       kFulfillReactionsOffset + kPointerSize;
   static const int kFlagsOffset = kRejectReactionsOffset + kPointerSize;
   static const int kSize = kFlagsOffset + kPointerSize;
+  static const int kSizeWithEmbedderFields =
+      kSize + v8::Promise::kEmbedderFieldCount * kPointerSize;
 
   // Flags layout.
   static const int kHasHandlerBit = 0;