Add switch to turn on 3P cookie blocking from command line

This switch is intended for testing purposes only and is not meant to
be enabled in production. It allows developers to test their sites
in order to adopt Privacy Sandbox APIs meant to replace third-party
cookies.

For now, the switch just enables 3PC blocking at startup and
overrides any changes to the 3P cookie blocking content setting.
Site-specific content settings can override this switch, which should
allow cookie access due to SAA.

When 3P cookie blocking is enabled, some Privacy Sandbox APIs are
disabled (e.g. FedCM). This is acceptable for now, but we should
iterate on this switch to enable these purpose-driven APIs and still
disables third-party cookie tracking.

Bug: 1441039
Change-Id: I59f2e1de06df542fb0dceb039c2b3ab98a43ae60
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/4544499


Reviewed-by: Maks Orlovich <morlovich@chromium.org>
Reviewed-by: Balazs Engedy <engedy@chromium.org>
Commit-Queue: Dylan Cutler <dylancutler@google.com>
Reviewed-by: Nasko Oskov <nasko@chromium.org>
Cr-Commit-Position: refs/heads/main@{#1147163}