Minor refactoring.

R=titzer@chromium.org

Bug: v8:8238
Change-Id: Ibf3388cf8fc4a8d618e2e0da53209e29e753058d
Reviewed-on: https://chromium-review.googlesource.com/c/1356501Reviewed-by: Ben Titzer <titzer@chromium.org>
Commit-Queue: Clemens Hammacher <clemensh@chromium.org>
Cr-Commit-Position: refs/heads/master@{#57958}