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This dialog had a two-phase construction to supply some state that
logically was part of construction but was not yet available at
construction time. This change collapses construction of this dialog
into a single logical phase. Specifically:

1) ProfileSigninConfirmationDialogViews::ShowDialog now defers
   construction of the dialog until after
   ui::CheckShouldPromptForNewProfile
1) CheckShouldPromptForNewProfile now takes a base::OnceCallback instead
   of a base::Callback, to allow ownership of the
   ProfileSigninConfirmationDelegate to be passed through it
3) ProfileSigninConfirmationDialogViews::Show now both constructs and
   shows the ProfileSigninConfirmationDialogViews, passing in the
   prompt_for_new_profile member
4) The prompt_for_new_profile member is now made const
5) That makes the return values of GetDialogButtonLabel also constant
   after construction, which opens up migration to set_button_label

Bug: 1011446
Change-Id: I975d425546db6e2d231f0003d38f0f3fac64ad88
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1869489Reviewed-by: Peter Boström <pbos@chromium.org>
Commit-Queue: Elly Fong-Jones <ellyjones@chromium.org>
Cr-Commit-Position: refs/heads/master@{#707480}