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This CL also delays creating the AuthenticatorRequestDialogModel and
showing the dialog until after ChromeAuthenticatorRequestDelegate's
OnTransportAvailabilityEnumerated is called, plus renames Step::kInitial
to ::kWelcome.

Bug: 847985
Change-Id: I3b46781ccea0309e1efc7c3fec45cbe4c43e9017
Reviewed-on: https://chromium-review.googlesource.com/1175120
Commit-Queue: Balazs Engedy <engedy@chromium.org>
Reviewed-by: Jun Choi <hongjunchoi@chromium.org>
Cr-Commit-Position: refs/heads/master@{#584041}