Spec change: https://github.com/tc39/proposal-bigint/pull/138

Bug: v8:6791
Cq-Include-Trybots: luci.v8.try:v8_linux_noi18n_rel_ng
Change-Id: I7367273ed1e98971be3b277f6486333a96412185
Reviewed-on: https://chromium-review.googlesource.com/1004120
Commit-Queue: Jakob Kummerow <jkummerow@chromium.org>
Reviewed-by: Georg Neis <neis@chromium.org>
Cr-Commit-Position: refs/heads/master@{#52611}