[bigint] Implement BigInt.prototype.toLocaleString

It does the same as .toString, which is "permissible but not encouraged"
per the spec and matches our behavior for Number.prototype.toString.

Bug: v8:6791
Change-Id: I25a565391abe0d055b8ef814214ecdad254f75e2
Reviewed-on: https://chromium-review.googlesource.com/917025Reviewed-by: Georg Neis <neis@chromium.org>
Commit-Queue: Jakob Kummerow <jkummerow@chromium.org>
Cr-Commit-Position: refs/heads/master@{#51296}

src/builtins/builtins-bigint.cc

@@ -70,14 +70,6 @@ BUILTIN(BigIntAsIntN) {
   return *BigInt::AsIntN(bits->Number(), bigint);
 }
 
-BUILTIN(BigIntPrototypeToLocaleString) {
-  HandleScope scope(isolate);
-
-  // TODO(jkummerow): Implement.
-
-  UNIMPLEMENTED();
-}
-
 namespace {
 
 MaybeHandle<BigInt> ThisBigIntValue(Isolate* isolate, Handle<Object> value,
@@ -100,18 +92,14 @@ MaybeHandle<BigInt> ThisBigIntValue(Isolate* isolate, Handle<Object> value,
       BigInt);
 }
 
-}  // namespace
-
-BUILTIN(BigIntPrototypeToString) {
-  HandleScope scope(isolate);
+Object* BigIntToStringImpl(Handle<Object> receiver, Handle<Object> radix,
+                           Isolate* isolate, const char* builtin_name) {
   // 1. Let x be ? thisBigIntValue(this value).
   Handle<BigInt> x;
   ASSIGN_RETURN_FAILURE_ON_EXCEPTION(
-      isolate, x,
-      ThisBigIntValue(isolate, args.receiver(), "BigInt.prototype.toString"));
+      isolate, x, ThisBigIntValue(isolate, receiver, builtin_name));
   // 2. If radix is not present, let radixNumber be 10.
   // 3. Else if radix is undefined, let radixNumber be 10.
-  Handle<Object> radix = args.atOrUndefined(isolate, 1);
   int radix_number;
   if (radix->IsUndefined(isolate)) {
     radix_number = 10;
@@ -131,6 +119,22 @@ BUILTIN(BigIntPrototypeToString) {
   RETURN_RESULT_OR_FAILURE(isolate, BigInt::ToString(x, radix_number));
 }
 
+}  // namespace
+
+BUILTIN(BigIntPrototypeToLocaleString) {
+  HandleScope scope(isolate);
+  Handle<Object> radix = args.atOrUndefined(isolate, 1);
+  return BigIntToStringImpl(args.receiver(), radix, isolate,
+                            "BigInt.prototype.toLocaleString");
+}
+
+BUILTIN(BigIntPrototypeToString) {
+  HandleScope scope(isolate);
+  Handle<Object> radix = args.atOrUndefined(isolate, 1);
+  return BigIntToStringImpl(args.receiver(), radix, isolate,
+                            "BigInt.prototype.toString");
+}
+
 BUILTIN(BigIntPrototypeValueOf) {
   HandleScope scope(isolate);
   RETURN_RESULT_OR_FAILURE(