[mathml] Implement display:contents for MathML

Make display:contents compute to display:none for MathML elements
as specified here:
https://drafts.csswg.org/css-display/#unbox-mathml

Bug: 6606
Change-Id: Ifb68940dd345a41ca5fe083dc5b5ba9ace1af506
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1903136
Reviewed-by: Frédéric Wang <fwang@igalia.com>
Reviewed-by: Rune Lillesveen <futhark@chromium.org>
Commit-Queue: Rob Buis <rbuis@igalia.com>
Cr-Commit-Position: refs/heads/master@{#713764}
2 files changed