This is based on https://crrev.com/c/867063.

R=titzer@chromium.org

Bug: chromium:798964
Change-Id: I3b019040da90acd9541b4ec5c88d07ae81007120
Reviewed-on: https://chromium-review.googlesource.com/876989Reviewed-by: Ben Titzer <titzer@chromium.org>
Commit-Queue: Eric Holk <eholk@chromium.org>
Cr-Commit-Position: refs/heads/master@{#50772}