cros: Simplify BaseScreen by eliminating OnShow/OnHide.

This was used only by ErrorScreen, and it is not clear what the
difference is supposed to be between the previous implementation style
(Show/Hide) and the new implementation style (OnShow/OnHide).

Also remove some other methods which were unused.

Change-Id: Ia31e05fc97a86f665f41abfeb8d4266aac3defc0
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1413750
Commit-Queue: Jacob Dufault <jdufault@chromium.org>
Reviewed-by: Alexander Alekseev <alemate@chromium.org>
Cr-Commit-Position: refs/heads/master@{#639720}
5 files changed