[css-display] Support pseudo-elements on display: contents elements.

This is split from https://codereview.chromium.org/2727853002/, since display:
contents ::before and ::after still need some extra work.

BUG=657748

Review-Url: https://codereview.chromium.org/2789283003
Cr-Commit-Position: refs/heads/master@{#461848}
3 files changed