mash: Move SystemTrayDelegate ownership to WmShell

ash/shell.h

Index: ash/shell.h
diff --git a/ash/shell.h b/ash/shell.h
index 6aff8bcc1d77d75b5e6ff611317a9aa03e4b1246..6e190e40106ae7d75a3c8cbb1e5a4796b1b95d31 100644
--- a/ash/shell.h
+++ b/ash/shell.h
@@ -495,10 +495,6 @@ class ASH_EXPORT Shell : public SystemModalContainerEventFilterDelegate,
   // Returns the system tray on primary display.
   SystemTray* GetPrimarySystemTray();
 
-  SystemTrayDelegate* system_tray_delegate() {
-    return system_tray_delegate_.get();
-  }
-
   SystemTrayNotifier* system_tray_notifier() {
     return system_tray_notifier_.get();
   }
@@ -665,7 +661,6 @@ class ASH_EXPORT Shell : public SystemModalContainerEventFilterDelegate,
   std::unique_ptr<UserMetricsRecorder> user_metrics_recorder_;
   std::unique_ptr<AcceleratorController> accelerator_controller_;
   std::unique_ptr<ShellDelegate> delegate_;
-  std::unique_ptr<SystemTrayDelegate> system_tray_delegate_;
   std::unique_ptr<SystemTrayNotifier> system_tray_notifier_;
   std::unique_ptr<UserWallpaperDelegate> user_wallpaper_delegate_;
   std::unique_ptr<SessionStateDelegate> session_state_delegate_;