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In the Account picker Dialog we had implemented an option for users
to sign in temporarily via Incognito mode.

This CL adds the functionality of showing an incognito interstitial when
the user clicks on the incognito mode button.

The incognito interstitial is currently blank expect the Learn more
and Continue buttons. The functionality of these buttons and the
contents of the interstitial would be followed up later.

This CL also adds tests to ensure the UI elements of the incognito
interstitial are shown properly and the state switch happened smoothly
and didn't leave any past UI elements.

TBR=bsazonov@chromium.org

Bug: 1103262
Change-Id: I031b505fe347afb3265da4921c002a702a022421
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2346645Reviewed-by: Rohit Agarwal <roagarwal@chromium.org>
Reviewed-by: Alice Wang <aliceywang@chromium.org>
Commit-Queue: Rohit Agarwal <roagarwal@chromium.org>
Cr-Commit-Position: refs/heads/master@{#798566}