Sync with latest Intl.RelativeTimeFormat spec.
See https://github.com/tc39/proposal-intl-relative-time/pull/99
See https://github.com/tc39/proposal-intl-relative-time/pull/100

Bug: v8:8613
Change-Id: Icc5bb73ecf65e979abc23cc430259584a7bf4b48
Reviewed-on: https://chromium-review.googlesource.com/c/1385930
Commit-Queue: Frank Tang <ftang@chromium.org>
Reviewed-by: Sathya Gunasekaran <gsathya@chromium.org>
Cr-Commit-Position: refs/heads/master@{#58607}