[bigint] Fix length of '0' sequences in fast .toString()

Bug: v8:11515
Change-Id: I1353726c9e81c3601258202fe56c05ffd16a4a25
Fixed: chromium:1232733
Reviewed-on: https://chromium-review.googlesource.com/c/v8/v8/+/3054112Reviewed-by: Marja Hölttä <marja@chromium.org>
Commit-Queue: Jakob Kummerow <jkummerow@chromium.org>
Cr-Commit-Position: refs/heads/master@{#75927}

src/bigint/tostring.cc

@@ -425,7 +425,7 @@ char* ToStringFormatter::ProcessLevel(RecursionLevel* level, Digits chunk,
     // Fill up with zeros up to the character count expected to be generated
     // on this level; unless this is the left edge of the result.
     if (is_last_on_level) return out;
-    int chunk_chars = level == nullptr ? chunk_chars_ : level->char_count_;
+    int chunk_chars = level == nullptr ? chunk_chars_ : level->char_count_ * 2;
     char* end = prev_cursor - chunk_chars;
     while (out != end) {
       *(--out) = '0';

test/mjsunit/harmony/bigint/regress-tostring.js

+// Copyright 2021 the V8 project authors. All rights reserved.
+// Use of this source code is governed by a BSD-style license that can be
+// found in the LICENSE file.
+
+// Regression test for crbug.com/1232733
+let big = 10n ** 900n;
+let expected = "1" + "0".repeat(900);
+assertEquals(expected, big.toString());