Stop the OverviewButton from appearing on the Add-User screen

ash/system/overview/overview_button_tray.h

 // Copyright 2014 The Chromium Authors. All rights reserved.
 // Use of this source code is governed by a BSD-style license that can be
 // found in the LICENSE file.
 
 #ifndef ASH_SYSTEM_OVERVIEW_OVERVIEW_BUTTON_TRAY_H_
 #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"
 
 namespace views {
 class ImageView;
 }
 
 namespace ash {
 
 // Status area tray for showing a toggle for Overview Mode. Overview Mode
 // is equivalent to WindowSelectorController being in selection mode.
 // This hosts a ShellObserver that listens for the activation of Maximize Mode
 // 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);
   ~OverviewButtonTray() override;
 
   // Updates the tray's visibility based on the LoginStatus and the current
   // state of MaximizeMode
   virtual void UpdateAfterLoginStatusChange(user::LoginStatus status);
 
   // ActionableView:
   bool PerformAction(const ui::Event& event) override;
 
+  // SessionStateObserver:
+  void SessionStateChanged(SessionStateDelegate::SessionState state) override;
+
   // ShellObserver:
   void OnMaximizeModeStarted() override;
   void OnMaximizeModeEnded() override;
   void OnOverviewModeEnded() override;
 
   // TrayBackgroundView:
   bool ClickedOutsideBubble() override;
   base::string16 GetAccessibleNameForTray() override;
   void HideBubbleWithView(const views::TrayBubbleView* bubble_view) override;
   void SetShelfAlignment(ShelfAlignment alignment) override;
@@ skipping 11 matching lines @@
 
   // Weak pointer, will be parented by TrayContainer for its lifetime.
   views::ImageView* icon_;
 
   DISALLOW_COPY_AND_ASSIGN(OverviewButtonTray);
 };
 
 }  // namespace ash
 
 #endif  // ASH_SYSTEM_OVERVIEW_OVERVIEW_BUTTON_TRAY_H_