R=mstarzinger@chromium.org

Bug: 
Change-Id: I95acea7b33a6e5799399d0891b2a52103f5e4964
Reviewed-on: https://chromium-review.googlesource.com/598072Reviewed-by: Ben Titzer <titzer@chromium.org>
Reviewed-by: Yang Guo <yangguo@chromium.org>
Reviewed-by: Ross McIlroy <rmcilroy@chromium.org>
Commit-Queue: Ben Titzer <titzer@chromium.org>
Cr-Commit-Position: refs/heads/master@{#47116}