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Add an InSessionAuthDialog class with a debug view, which currently
is a text prompt and two buttons. The InSessionAuthDialog will
show this debug view if the runtime flag show-auth-dialog-dev-overlay
is set, and will show nothing otherwise.

InSessionAuthDialog will be called by an AuthDialogController class
(to be added in a follow-up CL).ash: Add a barebone AuthenticationDialog class

Bug: b:156258540
Change-Id: Ib764399702ca557014b24faadf7855c4f9f39b07
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2298527
Commit-Queue: Yicheng Li <yichengli@chromium.org>
Reviewed-by: Xiyuan Xia <xiyuan@chromium.org>
Cr-Commit-Position: refs/heads/master@{#792404}