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Adds the "SameSite" level to the ProactivelySwapBrowsingInstance feature
flag. This CL also adds helper functions (to parametrize tests, etc.).
The implementation for same-site proactive BI swap is done in another
CL - crrev.com/c/2121522.
Non-content uses of the functions: crrev.com/c/2210605,
crrev.com/c/2196105, and a few others.

For more details, see doc: https://docs.google.com/document/d/1lHdkKLUe8H6ZP6ALwj-dsus7oYcuc93HkSCHCcerItg/edit?usp=sharing

Bug: 977562
Change-Id: I283ad31297ce6dd93bb6f02e8159488d62c5d989
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2214588Reviewed-by: Alex Moshchuk <alexmos@chromium.org>
Reviewed-by: Fergal Daly <fergal@chromium.org>
Reviewed-by: Alexander Timin <altimin@chromium.org>
Commit-Queue: Rakina Zata Amni <rakina@chromium.org>
Cr-Commit-Position: refs/heads/master@{#774980}