Add native callbacks to FastAccessorAssembler.

include/v8-experimental.h

Index: include/v8-experimental.h
diff --git a/include/v8-experimental.h b/include/v8-experimental.h
index f988e14054cec6275cd029a22cd759230f798a34..3874e91101f34799f99e932432272304201bfd56 100644
--- a/include/v8-experimental.h
+++ b/include/v8-experimental.h
@@ -39,6 +39,7 @@ class V8_EXPORT FastAccessorBuilder {
   LabelId MakeLabel();
   void SetLabel(LabelId label_id);
   void CheckNotZeroOrJump(ValueId value_id, LabelId label_id);
+  ValueId Call(v8::FunctionCallback callback, ValueId value_id);
 
  private:
   FastAccessorBuilder() = delete;