[mathml] Implement the <mspace> element

Implement the <mspace> element [1] by adding support for it
on the DOM and adding a layout algorithm for it. Also map attribute width
to the width property and map height and depth to the height property
using calc().

Bug: 6606

[1] https://mathml-refresh.github.io/mathml-core/#space-mspace

Change-Id: Ic52832edd6a73d896c2d42041756ded5d50fcb95
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1936251
Commit-Queue: Rob Buis <rbuis@igalia.com>
Reviewed-by: Ian Kilpatrick <ikilpatrick@chromium.org>
Reviewed-by: Frédéric Wang <fwang@igalia.com>
Reviewed-by: Morten Stenshorne <mstensho@chromium.org>
Reviewed-by: Rune Lillesveen <futhark@chromium.org>
Cr-Commit-Position: refs/heads/master@{#734439}
16 files changed