[bigint] Fix: toLocaleString does not take arguments

Bug: v8:6791
Change-Id: I43c43d217f00720ab666ff9908555fcd0fffe3b5
Reviewed-on: https://chromium-review.googlesource.com/919566Reviewed-by: Georg Neis <neis@chromium.org>
Commit-Queue: Jakob Kummerow <jkummerow@chromium.org>
Cr-Commit-Position: refs/heads/master@{#51314}

src/builtins/builtins-bigint.cc

@@ -123,7 +123,7 @@ Object* BigIntToStringImpl(Handle<Object> receiver, Handle<Object> radix,
 
 BUILTIN(BigIntPrototypeToLocaleString) {
   HandleScope scope(isolate);
-  Handle<Object> radix = args.atOrUndefined(isolate, 1);
+  Handle<Object> radix = isolate->factory()->undefined_value();
   return BigIntToStringImpl(args.receiver(), radix, isolate,
                             "BigInt.prototype.toLocaleString");
 }