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This CL introduces AuthenticatorSheetModelBase, which will serve as a
base class for sheets, and implement the shared behavior used on most
sheets, as well as maintaining a weak pointer to the dialog model.

Also introduce CreateSheetViewForCurrentStepOf, a factory method that
constructs the appropriate SheetView subclass, along with the
appropriate SheetModel subclass.

Bug: 849323
Change-Id: Iaedb3da6aa3f3fdc932b57cc5795143acab7d4c5
Reviewed-on: https://chromium-review.googlesource.com/1104422Reviewed-by: Kim Paulhamus <kpaulhamus@chromium.org>
Reviewed-by: Jun Choi <hongjunchoi@chromium.org>
Commit-Queue: Balazs Engedy <engedy@chromium.org>
Cr-Commit-Position: refs/heads/master@{#568114}