Replace NIK IsFrameSiteEnabled with GetMode

Replaces IsFrameSiteEnabled with GetMode(), per the suggestion here:
https://chromium-review.googlesource.com/c/chromium/src/+/4235742/comment/29617b56_aea09f18/

When we add support for testing 2.5 keys as well, this will allow us to
better express that only one of these schemes is enabled at a time, and
we'll update parameterized tests to make use of the new enum as well.

This CL doesn't introduce any behavioral changes.

Bug: 1414808
Change-Id: I242c5cd6c4e43053291f91b000dce64a8e43f7d3
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/4317883
Reviewed-by: Matt Menke <mmenke@chromium.org>
Reviewed-by: Ken Buchanan <kenrb@chromium.org>
Commit-Queue: Andrew Williams <awillia@chromium.org>
Cr-Commit-Position: refs/heads/main@{#1114828}