chromeos: Fix shelf appearing at login screen under mash

ash/aura/wm_shell_aura.h

Index: ash/aura/wm_shell_aura.h
diff --git a/ash/aura/wm_shell_aura.h b/ash/aura/wm_shell_aura.h
index 9795be15a9ff97cd3f94e9a5f6c2ee5931352bbb..d925a54724e117f3d4975dbb771eff25a4fabe2b 100644
--- a/ash/aura/wm_shell_aura.h
+++ b/ash/aura/wm_shell_aura.h
@@ -97,6 +97,9 @@ class ASH_EXPORT WmShellAura : public WmShell,
   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;