Reland cros: Views login backend for removing users.

Relands https://chromium-review.googlesource.com/c/chromium/src/+/935675. It was
reverted in https://chromium-review.googlesource.com/c/chromium/src/+/969228

TBR: xiyuan@chromium.org
TBR: dcheng@chromium.org
Bug: 809637
Change-Id: I9e44d96cbe26bc090e177460e18542b8b4d387b2
Reviewed-on: https://chromium-review.googlesource.com/971161
Reviewed-by: Jacob Dufault <jdufault@chromium.org>
Reviewed-by: Xiyuan Xia <xiyuan@chromium.org>
Commit-Queue: Jacob Dufault <jdufault@chromium.org>
Cr-Commit-Position: refs/heads/master@{#544423}

• 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/lock_contents_view.cc[diff]
• ash/login/ui/lock_contents_view.h[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/login/ui/login_bubble.cc[diff]
• ash/login/ui/login_bubble.h[diff]
• ash/login/ui/login_bubble_unittest.cc[diff]
• ash/login/ui/login_test_base.h[diff]
• ash/login/ui/login_test_utils.cc[diff]
• ash/login/ui/login_test_utils.h[diff]
• ash/login/ui/login_user_view.cc[diff]
• ash/login/ui/login_user_view.h[diff]
• ash/login/ui/login_user_view_unittest.cc[diff]
• ash/login/ui/scrollable_users_list_view.cc[diff]
• ash/login/ui/scrollable_users_list_view.h[diff]
• ash/metrics/login_metrics_recorder_unittest.cc[diff]
• ash/public/interfaces/login_screen.mojom[diff]
• chrome/browser/ui/ash/login_screen_client.cc[diff]
• chrome/browser/ui/ash/login_screen_client.h[diff]