Remove explicit numbering from ReauthAccessPoint

The numeric values don't need to be stable, they are not used for
metric buckets, pref values or anything of the sort. This CL makes
that fact clearer. Note that the integer values are consumed in
GetReauthAccessPointForReauthConfirmationURL(), but they are
generated by the browser itself in GetReauthConfirmationURL(), so
changing them should still work.

kMaxValue is also renamed to kMax because kMax cannot be used for
metric buckets.

Bug: None
Change-Id: I4338d3d3d916012bb92f4e8f6d8d3b71458a3e23
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/5942717
Auto-Submit: Victor Vianna <victorvianna@google.com>
Commit-Queue: Victor Vianna <victorvianna@google.com>
Reviewed-by: Alex Ilin <alexilin@chromium.org>
Cr-Commit-Position: refs/heads/main@{#1370702}