R=titzer@chromium.org

Bug: 
Change-Id: I2710aa5605bf2a26b6f86db98338dd54b6b87d2a
Reviewed-on: https://chromium-review.googlesource.com/600235
Commit-Queue: Andreas Rossberg <rossberg@chromium.org>
Reviewed-by: Ben Titzer <titzer@chromium.org>
Cr-Commit-Position: refs/heads/master@{#47140}