Notify login shelf when oobe ui dialog is hidden

When OOBE dialog is hidden, the login shelf should be notified, so it
can clear its cached dialog_state_. Otherwise, it will keep using the
cached dialog state when calculating shutdown button visibility, and
the button will remain hidden if the lock screen is shown later in the
session.

BUG=1051826

Change-Id: Id3fba45fd912a3eca2cc5b62b0e66f787d6e91d5
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2058999Reviewed-by: Denis Kuznetsov [CET] <antrim@chromium.org>
Commit-Queue: Toni Baržić <tbarzic@chromium.org>
Cr-Commit-Position: refs/heads/master@{#743138}

chrome/browser/chromeos/login/ui/login_display_host_webui.cc

@@ -887,6 +887,11 @@ void LoginDisplayHostWebUI::InitLoginWindowAndView() {
 }
 
 void LoginDisplayHostWebUI::ResetLoginWindowAndView() {
+  // Notify any oobe dialog state observers (e.g. login shelf) that the UI is
+  // hidden (so they can reset any cached OOBE dialog state.)
+  ash::LoginScreen::Get()->GetModel()->NotifyOobeDialogState(
+      ash::OobeDialogState::HIDDEN);
+
   // Make sure to reset the |login_view_| pointer first; it is owned by
   // |login_window_|. Closing |login_window_| could immediately invalidate the
   // |login_view_| pointer.