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Also adds doc and test around s2langquadtree's unique face constraint.

Bug: 850947
Change-Id: Ie8a48040bddeb011b59a8e3be230f1fdfa0d3406
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1553586
Commit-Queue: Alexandre Frechette <frechette@chromium.org>
Reviewed-by: anthonyvd <anthonyvd@chromium.org>
Cr-Commit-Position: refs/heads/master@{#650443}