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Stops using ScreenExitCode enums reported by enrollment screens (which
is part of a larger effort to deprecate ScreenExitCode enum all together).
Instead, defines result enums per screen when needed (i.e. if the screen
can report multiple values).

Updates how screens relay the exit code to WizardController - instead of
using BaseScreenDelegate::OnExit (indirectly though
BaseScreen::Finish), the screens now take a callback that is run when
the screen exits.

Affected screens:
  * EnrollmentScreen
  * AutoEnrollmentCheckScreen

Deprecated exit codes:
  * ENTERPRISE_ENROLLMENT_COMPLETED
  * ENTERPRISE_ENROLLMENT_BACK
  * ENTERPRISE_AUTO_ENROLLMENT_CHECK_COMPLETED

  BUG=930267

Change-Id: I2ade27b1a8d8a3b1e2a0993d76d3056c63feb61a
Reviewed-on: https://chromium-review.googlesource.com/c/1484079Reviewed-by: Jacob Dufault <jdufault@chromium.org>
Reviewed-by: Denis Kuznetsov <antrim@chromium.org>
Commit-Queue: Toni Baržić <tbarzic@chromium.org>
Cr-Commit-Position: refs/heads/master@{#636198}