Add unit tests for screen capture and share notification UI.

ash/system/chromeos/screen_security/screen_tray_item.h

Index: ash/system/chromeos/screen_security/screen_tray_item.h
diff --git a/ash/system/chromeos/screen_security/screen_tray_item.h b/ash/system/chromeos/screen_security/screen_tray_item.h
index ff7e1bbb8afa6b61ce831ee0fede5b117c82f666..03b8435d34f18498948c3eb238fda05554b24dd0 100644
--- a/ash/system/chromeos/screen_security/screen_tray_item.h
+++ b/ash/system/chromeos/screen_security/screen_tray_item.h
@@ -99,7 +99,7 @@ class ScreenNotificationView : public TrayNotificationView {
 // The base tray item for screen capture and screen sharing. The
 // Start method brings up a notification and a tray item, and the user
 // can stop the screen capture/sharing by pressing the stop button.
-class ScreenTrayItem : public SystemTrayItem {
+class ASH_EXPORT ScreenTrayItem : public SystemTrayItem {
  public:
   explicit ScreenTrayItem(SystemTray* system_tray);
   virtual ~ScreenTrayItem();