Bug 1626315 [wpt PR 22572] - [mathml] Implement the <munder>, <mover>, <munderover...

Bug 1626315 [wpt PR 22572] - [mathml] Implement the <munder>, <mover>, <munderover> elements, a=testonly

Automatic update from web-platform-tests
[mathml] Implement the <munder>, <mover>, <munderover> elements

Implement the <munder>, <mover>, <munderover> elements and add a layout
algorithm [1] covering all three elements. This patch does
not yet cover all details of the algorithm (accent, stretchy operators)
to keep the patch simple, however FIXMEs are added.

[1] https://mathml-refresh.github.io/mathml-core/#underscripts-and-overscripts-munder-mover-munderover

Bug: 6606
Change-Id: I8a20bb3105b733ef57413114aed0e6b536597c46
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2119538
Commit-Queue: Rob Buis <rbuis@igalia.com>
Reviewed-by: Ian Kilpatrick <ikilpatrick@chromium.org>
Reviewed-by: Morten Stenshorne <mstensho@chromium.org>
Reviewed-by: Mason Freed <masonfreed@chromium.org>
Cr-Commit-Position: refs/heads/master@{#756225}

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wpt-commits: fcff0768b41e6d13869c4a546c684d2e35bcb741
wpt-pr: 22572