Mojo API for launching the public session

This CL includes:
1. Add mojo api for launching the public session.
2. Preload the webui for the post login screens.

Bug: 809635
Change-Id: I8b3e3877f250ea193ec6a49f90f6a0f275bafbaf
Reviewed-on: https://chromium-review.googlesource.com/957309
Commit-Queue: Xiaoyin Hu <xiaoyinh@chromium.org>
Reviewed-by: Tom Sepez <tsepez@chromium.org>
Reviewed-by: Xiyuan Xia <xiyuan@chromium.org>
Reviewed-by: Jacob Dufault <jdufault@chromium.org>
Reviewed-by: Achuith Bhandarkar <achuith@chromium.org>
Cr-Commit-Position: refs/heads/master@{#545233}

• ash/login/login_screen_controller.cc[diff]
• ash/login/login_screen_controller.h[diff]
• ash/login/mock_login_screen_client.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/gaia_dialog_delegate.cc[diff]
• chrome/browser/chromeos/login/ui/gaia_dialog_delegate.h[diff]
• chrome/browser/chromeos/login/ui/login_display_host_views.cc[diff]
• chrome/browser/chromeos/login/ui/login_display_host_views.h[diff]
• chrome/browser/ui/ash/login_screen_client.cc[diff]
• chrome/browser/ui/ash/login_screen_client.h[diff]