cros: Add enrollment button for external binary auth

Bug: 890912
Change-Id: Ia55a93a2d1c2c956b95797ca175c3116a8d2b255
Reviewed-on: https://chromium-review.googlesource.com/c/1279359
Commit-Queue: Quan Nguyen <qnnguyen@chromium.org>
Reviewed-by: Dominick Ng <dominickn@chromium.org>
Reviewed-by: Jacob Dufault <jdufault@chromium.org>
Reviewed-by: Mitsuru Oshima <oshima@chromium.org>
Cr-Commit-Position: refs/heads/master@{#600837}

• ash/login/login_screen_controller.cc[diff]
• ash/login/login_screen_controller.h[diff]
• ash/login/mock_login_screen_client.cc[diff]
• ash/login/mock_login_screen_client.h[diff]
• ash/login/ui/login_auth_user_view.cc[diff]
• ash/login/ui/login_auth_user_view.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]