cros: Correctly handle login state on local accounts policy change

Hide Gaia screen when switches from 0 to >0 users, and show the
account picker screen otherwise.

Also remove can_close_ param from ShowGaiaSignin - relies on the user
list now.

Bug: 647451
Change-Id: Iec4e8182b6e07393a10d2336a9818d7d1b2d0b48
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2035670
Reviewed-by: Xiyuan Xia <xiyuan@chromium.org>
Reviewed-by: Denis Kuznetsov [CET] <antrim@chromium.org>
Commit-Queue: Roman Sorokin [CET] <rsorokin@chromium.org>
Cr-Commit-Position: refs/heads/master@{#743971}

diff --git a/ash/login/login_screen_controller.cc b/ash/login/login_screen_controller.cc
index 64b3432..1cba4cd 100644
--- a/ash/login/login_screen_controller.cc
+++ b/ash/login/login_screen_controller.cc

@@ -274,11 +274,16 @@
   client_->FocusLockScreenApps(reverse);
 }
 
-void LoginScreenController::ShowGaiaSignin(bool can_close,
-                                           const AccountId& prefilled_account) {
+void LoginScreenController::ShowGaiaSignin(const AccountId& prefilled_account) {
   if (!client_)
     return;
-  client_->ShowGaiaSignin(can_close, prefilled_account);
+  client_->ShowGaiaSignin(prefilled_account);
+}
+
+void LoginScreenController::HideGaiaSignin() {
+  if (!client_)
+    return;
+  client_->HideGaiaSignin();
 }
 
 void LoginScreenController::OnRemoveUserWarningShown() {