Add flag to chrome://flags to enable "SameSite by default cookies"

This flag will enable net::features::kSameSiteByDefaultCookies, which
will make cookies set without a valid SameSite attribute default to
being treated as SameSite=Lax. When the feature is enabled,
SameSite=None is required to get the previous default behavior of not
having any same-site restrictions.

Bug: 953306
Change-Id: Iaa7983e6568cdadf18752e89539c3e411325694e
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1574835
Reviewed-by: Maks Orlovich <morlovich@chromium.org>
Commit-Queue: Lily Chen <chlily@chromium.org>
Cr-Commit-Position: refs/heads/master@{#652863}