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This CL replaces the placeholder setup page with the real page which
includes the logic for choosing a multi-device host phone.

As part of this CL, WizardController now decides whether or not to
show the setup flow based on whether setup is applicable and needed for
the GAIA account under question.

Bug: 884065
Change-Id: I8f3be643b77890fb249516369d76f5c392a568a9
Reviewed-on: https://chromium-review.googlesource.com/c/1255591
Commit-Queue: Kyle Horimoto <khorimoto@chromium.org>
Reviewed-by: Xiyuan Xia <xiyuan@chromium.org>
Cr-Commit-Position: refs/heads/master@{#597178}