Given that we only support samesite lax/strict/none in our storage schema,
it's useful to introduce a default value, as required by the spec.
However, that would it hard to distinguish between none/lax when we switch
the default.
So, instead of doing that we use the peculiarities of our current schema
to our advantage: There's a "sameSite" attribute and a "rawSameSite"
attribute, where the latter is the literal value we received from the
server. With this patch, we'll interpret the "sameSite" attribute
based on the laxByDefault pref. This also has the advantage that various
front-end code (e.g., in DevTools) is always reading the "sameSite"
value of nsICookies.
Differential Revision: https://phabricator.services.mozilla.com/D137460
