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AuthenticatorRequestSheetView defines the basic structure of sheets
shown in the authenticator request dialog. Each sheet corresponds to a
certain step of the authentication flow, and encapsulates the controls
above the Ok/Cancel buttons, namely:
 -- an optional `back icon`,
 -- the title of the current step,
 -- the description of the current step, and
 -- a view with step-specific content, filling the rest of the space.

AuthenticatorRequestSheetModel will be subclassed for each step of
the authenticator request UX flow, and, for each step, the subclass is
going to encapsulate:

 (1) knowledge of the set of actions possible to the user at that step,

 (2) pieces of data required by views to visualise the sheet:
   (a) strings to use on labels/buttons rendered by the SheetView and
       the AuthenticatorRequestDialogView, and the state of the buttons,
   (b) data for additional step-specific contents rendered by SheetView
       subclasses, if any,

 (3) logic to handle user interactions with:
   (a) the `Back`, `Accept`, `Cancel`, buttons, even though the latter
       two are actually rendered by the AuthenticatorRequestDialogView,
   (b) the step-specific contents, if any.

For now, only a sheet with the original static content is shown.

Bug: 849323
Change-Id: I8b92f180a3dc3abc901dcb2ad0200e45271416a3
Reviewed-on: https://chromium-review.googlesource.com/1097321
Commit-Queue: Balazs Engedy <engedy@chromium.org>
Reviewed-by: Trent Apted <tapted@chromium.org>
Reviewed-by: Kim Paulhamus <kpaulhamus@chromium.org>
Cr-Commit-Position: refs/heads/master@{#567973}