Initial version of AuthenticationDialog.

Implement an initial MVP of the modal dialog that is shown during
WebAuthn requests. See the screenshot here: crbug.com/849323#c2.

The dialog is not yet interactive, even the `x` button does not yet
cancel the WebAuthn request (it does hide the dialog).

Bug: 849323
Change-Id: I3e1d9148f0727e33fb54e9bd1b747b84caa3c827
Reviewed-on: https://chromium-review.googlesource.com/964448
Commit-Queue: Balazs Engedy <[email protected]>
Reviewed-by: Trent Apted <[email protected]>
Cr-Commit-Position: refs/heads/master@{#565607}
16 files changed