[CSN] Uniquify URLs within a group

URLs are sometimes duplicated within a group. Make them unique within a
group by taking the first instance.

Change-Id: I7ae9be08a29783104e6852d02ac9506c1b407fdf
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2877695


Reviewed-by: Yashar Dabiran <yashard@chromium.org>
Commit-Queue: Calder Kitagawa <ckitagawa@chromium.org>
Cr-Commit-Position: refs/heads/master@{#880987}