[Partitioned Popins] Implement popinContextType() JS API
This CL implements window.popinContextType() that tells whether a page
is inside of a Partitioned Popin.
Explainer: https://explainers-by-googlers.github.io/partitioned-popins/
I2P: https://groups.google.com/a/chromium.org/g/blink-dev/c/ApU_zUmpQ2g/
Low-Coverage-Reason: COVERAGE_UNDERREPORTED Tested through WPTs.
Bug: 340606651
Bug: b/362973178
Change-Id: I053512217160d4875e7558be35eda3a3bbace833
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/5826165
Reviewed-by: Ari Chivukula arichiv@chromium.org
Reviewed-by: Robert Kaplow rkaplow@chromium.org
Reviewed-by: Daniel Cheng dcheng@chromium.org
Commit-Queue: Sandor «Alex» Major sandormajor@google.com
Cr-Commit-Position: refs/heads/main@{#1350877}