Mojo api for requesting keyboard layouts for public account user.

The requested keyboard layouts will be used in public session UI to
populate a selection menu. (In a follow-up CL)

Bug: 809635
Change-Id: I1154647d4bfdf3637ec37033f4452742c71ec377
Reviewed-on: https://chromium-review.googlesource.com/1020664
Reviewed-by: Xiyuan Xia <xiyuan@chromium.org>
Reviewed-by: Tom Sepez <tsepez@chromium.org>
Reviewed-by: Achuith Bhandarkar <achuith@chromium.org>
Commit-Queue: Xiaoyin Hu <xiaoyinh@chromium.org>
Cr-Commit-Position: refs/heads/master@{#554517}

• ash/login/login_screen_controller.cc[diff]
• ash/login/login_screen_controller.h[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_debug_view.cc[diff]
• ash/login/ui/login_data_dispatcher.cc[diff]
• ash/login/ui/login_data_dispatcher.h[diff]
• ash/public/interfaces/login_screen.mojom[diff]
• ash/public/interfaces/login_user_info.mojom[diff]
• chrome/browser/chromeos/login/screens/user_selection_screen.cc[diff]
• chrome/browser/chromeos/login/screens/user_selection_screen.h[diff]
• chrome/browser/chromeos/login/user_board_view_mojo.cc[diff]
• chrome/browser/ui/ash/login_screen_client.cc[diff]
• chrome/browser/ui/ash/login_screen_client.h[diff]