chromeos: Fix shelf appearing at login screen under mash

ash/aura/wm_shell_aura.h

 // Copyright 2016 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_AURA_WM_SHELL_AURA_H_
 #define ASH_AURA_WM_SHELL_AURA_H_
 
 #include <memory>
 
 #include "ash/ash_export.h"
@@ skipping 79 matching lines @@
 #endif
 
  private:
   // aura::client::ActivationChangeObserver:
   void OnWindowActivated(ActivationReason reason,
                          aura::Window* gained_active,
                          aura::Window* lost_active) override;
   void OnAttemptToReactivateWindow(aura::Window* request_active,
                                    aura::Window* actual_active) override;
 
+  // SessionStateObserver:
+  void SessionStateChanged(session_manager::SessionState state) override;
+
   // WindowTreeHostManager::Observer:
   void OnDisplayConfigurationChanging() override;
   void OnDisplayConfigurationChanged() override;
 
   WmLookupAura wm_lookup_;
   std::unique_ptr<PointerWatcherAdapter> pointer_watcher_adapter_;
 
   bool added_activation_observer_ = false;
   base::ObserverList<WmActivationObserver> activation_observers_;
 
   bool added_display_observer_ = false;
   base::ObserverList<WmDisplayObserver> display_observers_;
 
   DISALLOW_COPY_AND_ASSIGN(WmShellAura);
 };
 
 }  // namespace ash
 
 #endif  // ASH_AURA_WM_SHELL_AURA_H_