merge_pr_40

[css-pseudo] Fix assert failure when combining ::first-line and ::marker

The interaction of a ::marker pseudo-element with a 'content' property
and the ::first-line of the list-item produced a DCHECK failure in debug
builds (it was fine with debug disabled). This patch fixes it.

Bug: 457718

TEST=external/wpt/css/css-pseudo/first-line-and-marker.html

Change-Id: I443934f3a5cbfd95913287ac2bfb0966fa707e3d
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1937068
Reviewed-by: Rune Lillesveen futhark@chromium.org
Reviewed-by: Koji Ishii kojii@chromium.org
Commit-Queue: Oriol Brufau obrufau@igalia.com
Cr-Commit-Position: refs/heads/master@{#719561}