cros: UI for unlocking with an external binary.

See the bug for more details.

Bug: 887070
Change-Id: I4218ffc18f64462c79be31d453d5198003c5d540
Reviewed-on: https://chromium-review.googlesource.com/1235124
Reviewed-by: Dominick Ng <dominickn@chromium.org>
Reviewed-by: Xiyuan Xia <xiyuan@chromium.org>
Commit-Queue: Jacob Dufault <jdufault@chromium.org>
Cr-Commit-Position: refs/heads/master@{#592969}

• ash/login/login_screen_controller.cc[diff]
• ash/login/login_screen_controller.h[diff]
• ash/login/login_screen_controller_unittest.cc[diff]
• ash/login/mock_login_screen_client.cc[diff]
• ash/login/mock_login_screen_client.h[diff]
• ash/login/ui/lock_contents_view.cc[diff]
• ash/login/ui/lock_contents_view_unittest.cc[diff]
• ash/login/ui/lock_screen_sanity_unittest.cc[diff]
• ash/login/ui/login_auth_user_view.cc[diff]
• ash/login/ui/login_auth_user_view.h[diff]
• ash/login/ui/login_auth_user_view_unittest.cc[diff]
• ash/public/cpp/ash_features.cc[diff]
• ash/public/cpp/ash_features.h[diff]
• ash/public/interfaces/login_screen.mojom[diff]
• chrome/browser/chromeos/login/lock/views_screen_locker.cc[diff]
• chrome/browser/chromeos/login/lock/views_screen_locker.h[diff]
• chrome/browser/chromeos/login/ui/login_display_host_mojo.cc[diff]
• chrome/browser/chromeos/login/ui/login_display_host_mojo.h[diff]
• chrome/browser/ui/ash/login_screen_client.cc[diff]
• chrome/browser/ui/ash/login_screen_client.h[diff]