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This is the refactor portion of https://crrev.com/c/1272085.

The logic in wizard_controller.cc is very complex. We should try to
move the screen specific logic out of it.

Bug: NONE
Change-Id: Id89479b4b145b2a80333bbbadd1e51472fb8ab49
Reviewed-on: https://chromium-review.googlesource.com/c/1281171
Commit-Queue: Wenzhao (Colin) Zang <wzang@chromium.org>
Reviewed-by: Alexander Alekseev <alemate@chromium.org>
Cr-Commit-Position: refs/heads/master@{#599872}