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The CSSWG would like to apply align-content to single-line flexboxes
as well:
https://lists.w3.org/Archives/Public/www-style/2018Nov/0007.html
https://github.com/w3c/csswg-drafts/issues/3052

Add a use counter for that to see if it is web-compatible.

TESTED=Manually using chrome://histograms
R=eae@chromium.org, tabatkins@google.com

Change-Id: I3af67474937c8c589e8235c451b358b839556d1f
Reviewed-on: https://chromium-review.googlesource.com/c/1354539
Commit-Queue: Christian Biesinger <cbiesinger@chromium.org>
Commit-Queue: Emil A Eklund <eae@chromium.org>
Reviewed-by: Emil A Eklund <eae@chromium.org>
Cr-Commit-Position: refs/heads/master@{#611984}