[IP Protection] Enable IP Protection by default for managed devices

From talking with the enterprise team, it'd be better not to exclude
enterprise users from this feature particularly since it only affects
incognito mode.

Note that the IP Protection enterprise policy had not been launched yet,
or else this would have been a disruptive change.

This CL launches the enterprise policy with a starting milestone of
M138.

Bug: 41496985
Change-Id: I30f01b8232831d2a56d9da3f420a33f7eabbe6c6
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/6488322
Commit-Queue: Andrew Williams <awillia@chromium.org>
Reviewed-by: David Schinazi <dschinazi@chromium.org>
Reviewed-by: Fiona Macintosh <fmacintosh@google.com>
Reviewed-by: Victor Gabriel Savu <vsavu@google.com>
Cr-Commit-Position: refs/heads/main@{#1457529}