Add unittests of ArcCustomNotificationView

ui/message_center/views/custom_notification_view.h

Index: ui/message_center/views/custom_notification_view.h
diff --git a/ui/message_center/views/custom_notification_view.h b/ui/message_center/views/custom_notification_view.h
index 0700010253ee5b28ea2feac390fe1719ea79e365..c9b000ad85624ba525c32b07fa8416a191004766 100644
--- a/ui/message_center/views/custom_notification_view.h
+++ b/ui/message_center/views/custom_notification_view.h
@@ -43,6 +43,8 @@ class MESSAGE_CENTER_EXPORT CustomNotificationView : public MessageView {
   bool OnKeyPressed(const ui::KeyEvent& event) override;
   void ChildPreferredSizeChanged(View* child) override;
 
+  views::View* GetContentsViewForTesting() const { return contents_view_; }
+
  private:
   friend class CustomNotificationViewTest;