Allow embedder to set promise internal field count

BUILD.gn

Index: BUILD.gn
diff --git a/BUILD.gn b/BUILD.gn
index 14c7a696e651f583b47d88d243b45a337735270d..6493571daa0e18bf6ebb3bc7d8af61f6032ba565 100644
--- a/BUILD.gn
+++ b/BUILD.gn
@@ -45,6 +45,9 @@ declare_args() {
   # Sets -dENABLE_DISASSEMBLER.
   v8_enable_disassembler = ""
 
+  # Sets the number of internal fields on promise objects.
+  v8_promise_internal_field_count = 0
+
   # Sets -dENABLE_GDB_JIT_INTERFACE.
   v8_enable_gdbjit = ""
 
@@ -206,6 +209,10 @@ config("features") {
   if (v8_enable_disassembler) {
     defines += [ "ENABLE_DISASSEMBLER" ]
   }
+  if (v8_promise_internal_field_count != 0) {
+    defines +=
+        [ "V8_PROMISE_INTERNAL_FIELD_COUNT=${v8_promise_internal_field_count}" ]
+  }
   if (v8_enable_future) {
     defines += [ "V8_ENABLE_FUTURE" ]
   }