Discussed in
https://github.com/w3c/csswg-drafts/issues/492

Change-Id: I31172814ad0db99b2ede246f92c4611dab07c756
Reviewed-on: https://chromium-review.googlesource.com/759918
Commit-Queue: Eric Willigers <[email protected]>
Reviewed-by: Darren Shen <[email protected]>
Cr-Commit-Position: refs/heads/master@{#515117}