Refactor AddScrollListItem() in system menu detailed views

ash/system/tray/tray_details_view_unittest.cc

Index: ash/system/tray/tray_details_view_unittest.cc
diff --git a/ash/system/tray/tray_details_view_unittest.cc b/ash/system/tray/tray_details_view_unittest.cc
index 49d3f3a0dd333eb8066730bfbc95bec1aebe0301..cd13603e95e2b4de3d60c90c2fa347b56b436276 100644
--- a/ash/system/tray/tray_details_view_unittest.cc
+++ b/ash/system/tray/tray_details_view_unittest.cc
@@ -43,6 +43,10 @@ class TestDetailsView : public TrayDetailsView {
 
   void CreateScrollerViews() { CreateScrollableList(); }
 
+  views::View* scroll_content() const {
+    return TrayDetailsView::scroll_content();
+  }
+
  private:
   TrayPopupHeaderButton* tray_popup_header_button_;