Stop the OverviewButton from appearing on the Add-User screen

ash/system/overview/overview_button_tray.h

Index: ash/system/overview/overview_button_tray.h
diff --git a/ash/system/overview/overview_button_tray.h b/ash/system/overview/overview_button_tray.h
index a58bae11576457979207ee82e858ef9781a91a46..94da9e8073086c3dc82780ca91b6852ebb660491 100644
--- a/ash/system/overview/overview_button_tray.h
+++ b/ash/system/overview/overview_button_tray.h
@@ -6,6 +6,7 @@
 #define ASH_SYSTEM_OVERVIEW_OVERVIEW_BUTTON_TRAY_H_
 
 #include "ash/ash_export.h"
+#include "ash/session/session_state_observer.h"
 #include "ash/shell_observer.h"
 #include "ash/system/tray/tray_background_view.h"
 #include "ash/wm/overview/window_selector_controller.h"
@@ -22,6 +23,7 @@ namespace ash {
 // This tray will only be visible while in this state. This tray does not
 // provide any bubble view windows.
 class ASH_EXPORT OverviewButtonTray : public TrayBackgroundView,
+                                      public SessionStateObserver,
                                       public ShellObserver {
  public:
   explicit OverviewButtonTray(StatusAreaWidget* status_area_widget);
@@ -34,6 +36,9 @@ class ASH_EXPORT OverviewButtonTray : public TrayBackgroundView,
   // ActionableView:
   bool PerformAction(const ui::Event& event) override;
 
+  // SessionStateObserver:
+  void SessionStateChanged(SessionStateDelegate::SessionState state) override;
+
   // ShellObserver:
   void OnMaximizeModeStarted() override;
   void OnMaximizeModeEnded() override;