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We can just get them from the line. This also moves overriding the line's
cross axis extent to a more logical place (and adds it to the NG implementation).

Change-Id: I04da17674645fa55ff1903a15bdd4781ad26ef6e
Reviewed-on: https://chromium-review.googlesource.com/c/1338478Reviewed-by: David Grogan <dgrogan@chromium.org>
Commit-Queue: Christian Biesinger <cbiesinger@chromium.org>
Cr-Commit-Position: refs/heads/master@{#609022}