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The CSS Containment spec explicitly states that containment has no
effect on elements with display:contents element for paint, layout and
others. Not stated for contain:style.

Check for style containment for counters even when LayoutObject is null.

Note that due to the bug reported in 766650, the added test passes even
without this code change.

Bug: 766650
Change-Id: I64082f52255d5d810ba803148a418edff5da66c8
Reviewed-on: https://chromium-review.googlesource.com/771830Reviewed-by: Morten Stenshorne <mstensho@chromium.org>
Commit-Queue: Rune Lillesveen <futhark@chromium.org>
Cr-Commit-Position: refs/heads/master@{#517140}