[Popins] Implement Web Platform Test for popin partitions
This test creates same-site and cross-site iframes and popins in
different combinations to test that popins provide access to the proper
cookie and storage partitions.
Bug: 340606651
Bug: b/365144247
Change-Id: I377eb4066caf2d0491559f4d691c4e445c6a2409
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/5841474
Commit-Queue: Aaron Selya selya@google.com
Reviewed-by: Ari Chivukula arichiv@chromium.org
Reviewed-by: Aaron Selya selya@google.com
Cr-Commit-Position: refs/heads/main@{#1355425}