Sync w/ https://github.com/tc39/proposal-intl-segmenter/pull/60

Bug: v8:8717
Change-Id: I98fe9e88367a611c14c82195222c8fe8a52e4bc8
Reviewed-on: https://chromium-review.googlesource.com/c/1422749Reviewed-by: Jakob Kummerow <jkummerow@chromium.org>
Reviewed-by: Sathya Gunasekaran <gsathya@chromium.org>
Commit-Queue: Frank Tang <ftang@chromium.org>
Cr-Commit-Position: refs/heads/master@{#59016}