Use narrower types in CookiePartitionKeyTest

Some of the test case structs have a boolean field, and other fields
that are only meaningful if the aforementioned boolean is true.
Instead of using a bool and just ignoring other fields sometimes, we
should use an optional to make the semantics clearer and harder to
misuse.

This CL also adds some `using enum` directives to reduce noisy
namespacing.

Change-Id: I0ae5a60586d8e31de598616500fa1f78f98db45b
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/5466048


Reviewed-by: Dylan Cutler <dylancutler@google.com>
Commit-Queue: Chris Fredrickson <cfredric@chromium.org>
Auto-Submit: Chris Fredrickson <cfredric@chromium.org>
Cr-Commit-Position: refs/heads/main@{#1289585}