[Partitioned Popins] Sec-Popin-Context Request Header
This CL implements the HTTP Request Header Sec-Popin-Context which is
sent on top-level navigations/redirects within a popin.
Explainer: https://explainers-by-googlers.github.io/partitioned-popins/
I2P: https://groups.google.com/a/chromium.org/g/blink-dev/c/ApU_zUmpQ2g/
Bug: 340606651
Change-Id: I25ad7dd85197b549da5bf1e6d117edc7db6ffc96
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/5836978
Commit-Queue: Ari Chivukula arichiv@chromium.org
Reviewed-by: Rakina Zata Amni rakina@chromium.org
Cr-Commit-Position: refs/heads/main@{#1351341}