This is a follow-up cleanup to treat NumberRound like the other rounding
operations (NumberFloor, NumberCeil and NumberTrunc).

Bug: v8:8015
Change-Id: I2b2fbc7f0319497d16ccb7472595eeb68be1f51d
Reviewed-on: https://chromium-review.googlesource.com/c/1260403Reviewed-by: Sigurd Schneider <sigurds@chromium.org>
Commit-Queue: Benedikt Meurer <bmeurer@chromium.org>
Cr-Commit-Position: refs/heads/master@{#56371}