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Removes update screen related exit codes from ScreenExitCode enum, and
adds a separate enum to UpdateScreen class.

Error exit codes are condensed into a single UPDATE_ERROR code (as the
action taken by wizard controller is pretty much the same for all), and
the decidion whether non-critical updates should be ignored is moved
from WizardController to UpdateScreen - i.e. for non critical updates,
UpdateScreen will return UPDATE_NOT_REQUIRED result instead of an error.

Changes how UpdateScreen reports exit - instead of calling the
BaseScreenDelegate::OnExit, it runs a callback that is provided in its
constructor.

BUG=930267

Change-Id: I74b9c4c87f213eafbe6adb98f8942d4d8450dc09
Reviewed-on: https://chromium-review.googlesource.com/c/1481852Reviewed-by: Jacob Dufault <jdufault@chromium.org>
Reviewed-by: Alexander Alekseev <alemate@chromium.org>
Commit-Queue: Toni Baržić <tbarzic@chromium.org>
Cr-Commit-Position: refs/heads/master@{#635250}