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This is a reland of 430b7621

Original change's description:
> [LayoutNG] Don't truncate margins after spanners.
>
> A column spanner forcefully breaks a column, so any leading margins on
> content right after the spanner shouldn't be discarded.
>
> Bug: 829028
> Change-Id: I767466677d72be2760306ddd531e847f52cfa8ca
> Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2445482
> Reviewed-by: Alison Maher <almaher@microsoft.com>
> Reviewed-by: Ian Kilpatrick <ikilpatrick@chromium.org>
> Commit-Queue: Morten Stenshorne <mstensho@chromium.org>
> Cr-Commit-Position: refs/heads/master@{#813892}

Bug: 829028
Change-Id: I83b1242b9a2e47b7205fc35e31b806e370141e8d
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2454050Reviewed-by: Alison Maher <almaher@microsoft.com>
Commit-Queue: Morten Stenshorne <mstensho@chromium.org>
Cr-Commit-Position: refs/heads/master@{#814592}