ash: Add a unit test for the system update tray item

ash/system/tray_update.h

 // Copyright (c) 2012 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_TRAY_UPDATE_H_
 #define ASH_SYSTEM_TRAY_UPDATE_H_
 
+#include "ash/ash_export.h"
 #include "ash/common/system/update/update_observer.h"
 #include "ash/system/tray/tray_image_item.h"
 #include "base/macros.h"
 
 namespace views {
 class View;
 }
 
 namespace ash {
 
-class TrayUpdate : public TrayImageItem,
-                   public UpdateObserver {
+// The system update tray item. Exported for test.
+class ASH_EXPORT TrayUpdate : public TrayImageItem, public UpdateObserver {
  public:
   explicit TrayUpdate(SystemTray* system_tray);
   ~TrayUpdate() override;
 
  private:
   // Overridden from TrayImageItem.
   bool GetInitialVisibility() override;
   views::View* CreateDefaultView(LoginStatus status) override;
 
   // Overridden from UpdateObserver.
   void OnUpdateRecommended(const UpdateInfo& info) override;
 
   DISALLOW_COPY_AND_ASSIGN(TrayUpdate);
 };
 
 }  // namespace ash
 
 #endif  // ASH_SYSTEM_TRAY_UPDATE_H_