Create LoginShelfView for views-based lock screen

1) Hide the views-based shelf behind --show-md-login.

2) Only support lock/add user screens. Will support login screen when
views-based login is in place.

3) The buttons do not match the spec yet, and there's known issue with
focus and accessibility.

Bug: 701157
Change-Id: If6462f90a80f62f8fbca32674d30757fd48aecb5
Reviewed-on: https://chromium-review.googlesource.com/583942
Commit-Queue: Wenzhao (Colin) Zang <wzang@chromium.org>
Reviewed-by: James Cook <jamescook@chromium.org>
Reviewed-by: Oliver Chang <ochang@chromium.org>
Reviewed-by: Michael Wasserman <msw@chromium.org>
Reviewed-by: Jacob Dufault <jdufault@chromium.org>
Cr-Commit-Position: refs/heads/master@{#502462}

diff --git a/ash/login/lock_screen_controller.cc b/ash/login/lock_screen_controller.cc
index 14b8f35..b885fcf 100644
--- a/ash/login/lock_screen_controller.cc
+++ b/ash/login/lock_screen_controller.cc

@@ -166,6 +166,12 @@
   lock_screen_client_->SignOutUser();
 }
 
+void LockScreenController::CancelAddUser() {
+  if (!lock_screen_client_)
+    return;
+  lock_screen_client_->CancelAddUser();
+}
+
 void LockScreenController::OnMaxIncorrectPasswordAttempted(
     const AccountId& account_id) {
   if (!lock_screen_client_)