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Stops using remaining ScreenExitCode enum values (for reporing oobe screen
results. Instead, defines screen result enums per screen when needed (i.e.
if the screen may 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:
  * AppDownloadingScreen
  * ArcTermsOfServiceScreen
  * AssistantOptInScreen
  * DemoPreferencesScreen
  * DemoSetupScreen
  * DiscoverScreen
  * EnableDebuggingScreen
  * FingerprintSetupScreen
  * HIDDetectionScreen
  * KioskAutolaunchScreen
  * KioskEnableScreen
  * MarketingOptInScreen
  * MultideviceSetupScreen
  * RecommendAppsScreen
  * ResetScreen
  * SupervisionTransitionScreen
  * SyncConsentScreen
  * TermsOfServiceScreen
  * WelcomeScreen
  * WrongHWIDScreen

BUG=930267

Change-Id: Ie53a748b24a9bf8d03bc0e8e467d94d1f38e5898
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1506492
Commit-Queue: Toni Baržić <tbarzic@chromium.org>
Reviewed-by: Jacob Dufault <jdufault@chromium.org>
Cr-Commit-Position: refs/heads/master@{#638428}