This removes the last instance where we did manually what {LowerInt64}
already does.

R=titzer@chromium.org

Change-Id: I5e0de0f8ed1ae31f295e947a9fe36e4142200b65
Reviewed-on: https://chromium-review.googlesource.com/602238Reviewed-by: Ben Titzer <titzer@chromium.org>
Commit-Queue: Clemens Hammacher <clemensh@chromium.org>
Cr-Commit-Position: refs/heads/master@{#47217}