CSS: ellipse() has 0 or 2 radii

We were incorrectly accepting
"ellipse(10px at 20px 30px)"
as an ellipse.

Retire use count BasicShapeEllipseOneRadius 0.00002%
https://chromestatus.com/metrics/feature/timeline/popularity/2327

We were incorrectly serializing
"ellipse(10px closest-side at 20px 30px)" as
"ellipse(10px at 20px 30px)"

BUG=899150

Change-Id: I6594ff990f659be2497acb336afbb138b5d91ac8
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1593882
Commit-Queue: Emil A Eklund <eae@chromium.org>
Reviewed-by: Emil A Eklund <eae@chromium.org>
Cr-Commit-Position: refs/heads/master@{#656441}