[json] Allow any callable object for toJSON.

src/json-stringifier.h

Index: src/json-stringifier.h
diff --git a/src/json-stringifier.h b/src/json-stringifier.h
index 44e710375d43879a7f1b4dc10edc4d510343f532..b40a78249ff2f3a192d5c27aa8312e1d4f74bd82 100644
--- a/src/json-stringifier.h
+++ b/src/json-stringifier.h
@@ -245,7 +245,7 @@ MaybeHandle<Object> BasicJsonStringifier::ApplyToJsonFunction(
                     LookupIterator::PROTOTYPE_CHAIN_SKIP_INTERCEPTOR);
   Handle<Object> fun;
   ASSIGN_RETURN_ON_EXCEPTION(isolate_, fun, Object::GetProperty(&it), Object);
-  if (!fun->IsJSFunction()) return object;
+  if (!fun->IsCallable()) return object;
 
   // Call toJSON function.
   if (key->IsSmi()) key = factory()->NumberToString(key);