R=titzer@chromium.org

Bug: v8:5815
Change-Id: Ied267a431ed294f022628169e811069f2a4851b1
Reviewed-on: https://chromium-review.googlesource.com/599875
Commit-Queue: Andreas Rossberg <rossberg@chromium.org>
Reviewed-by: Ben Titzer <titzer@chromium.org>
Cr-Commit-Position: refs/heads/master@{#47133}