R=rossberg@chromium.org

Bug: 
Change-Id: Icac33dc87dd660173e5a45d02b31be46f7d1cb2d
Reviewed-on: https://chromium-review.googlesource.com/721550
Commit-Queue: Ben Titzer <titzer@chromium.org>
Reviewed-by: Andreas Rossberg <rossberg@chromium.org>
Cr-Commit-Position: refs/heads/master@{#48632}