Mash cleanup: migrate LoginScreenClient from mojo to C++.

LoginScreenClient becomes a C++ interface, and mojom::LoginScreen can
now be removed.

LoginScreen's last two methods, ShowLoginScreen and ShowLockScreen,
are moved to the C++ interface, and since they're now synchronous,
calling code has to be rearranged slightly. The return values are
removed, because ShowLockScreen always returned true and while
ShowLoginScreen sometimes returned false, the calling code always
CHECKed that the value was true.

TBR=tsepez@chromium.org

Bug: 958206
Change-Id: I650d5cec4f9b01e97a093d9d6180dddb170c46e2
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1643351
Reviewed-by: Evan Stade <estade@chromium.org>
Reviewed-by: Xiyuan Xia <xiyuan@chromium.org>
Commit-Queue: Evan Stade <estade@chromium.org>
Cr-Commit-Position: refs/heads/master@{#666980}

• 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.h[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_unittest.cc[diff]
• ash/login/ui/login_data_dispatcher.h[diff]
• ash/login/ui/login_expanded_public_account_view_unittest.cc[diff]
• ash/login/ui/login_test_base.cc[diff]
• ash/login/ui/login_test_base.h[diff]
• ash/login/ui/parent_access_view.cc[diff]
• ash/login/ui/parent_access_view.h[diff]
• ash/login/ui/parent_access_view_unittest.cc[diff]
• ash/mojo_interface_factory.cc[diff]
• ash/public/cpp/BUILD.gn[diff]
• ash/public/cpp/login_screen.h[diff]
• ash/public/cpp/login_screen_client.h[Added - diff]
• ash/public/cpp/login_types.h[diff]
• ash/public/cpp/manifest.cc[diff]
• ash/public/interfaces/BUILD.gn[diff]
• ash/public/interfaces/login_screen.mojom[Deleted - diff]
• ash/shelf/login_shelf_view.cc[diff]
• ash/shelf/login_shelf_view.h[diff]
• ash/shelf/login_shelf_view_unittest.cc[diff]
• ash/system/unified/unified_system_tray_controller.cc[diff]
• chrome/browser/chromeos/login/lock/screen_locker.cc[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/fake_login_display_host.cc[diff]
• chrome/browser/chromeos/login/ui/fake_login_display_host.h[diff]
• chrome/browser/chromeos/login/ui/login_display_host.h[diff]
• chrome/browser/chromeos/login/ui/login_display_host_common.cc[diff]
• chrome/browser/chromeos/login/ui/login_display_host_common.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/chromeos/login/ui/login_display_host_webui.cc[diff]
• chrome/browser/chromeos/login/ui/login_display_host_webui.h[diff]
• chrome/browser/chromeos/login/ui/login_display_mojo.cc[diff]
• chrome/browser/chromeos/login/ui/login_display_mojo.h[diff]
• chrome/browser/chromeos/login/ui/mock_login_display_host.h[diff]
• chrome/browser/ui/ash/login_screen_client.cc[diff]
• chrome/browser/ui/ash/login_screen_client.h[diff]
• chrome/browser/ui/ash/test_login_screen.cc[diff]
• chrome/browser/ui/ash/test_login_screen.h[diff]
• chrome/browser/ui/webui/chromeos/login/gaia_screen_handler.cc[diff]
• chrome/browser/ui/webui/chromeos/login/gaia_screen_handler.h[diff]