Remove unused references to NativeTheme in TrayPopupItemStyle.

ash/common/system/chromeos/screen_security/screen_tray_item.cc

Index: ash/common/system/chromeos/screen_security/screen_tray_item.cc
diff --git a/ash/common/system/chromeos/screen_security/screen_tray_item.cc b/ash/common/system/chromeos/screen_security/screen_tray_item.cc
index 4f5f73cb9ce12d2738199a24c485b359ad45ebef..aff5edc1276ccd5049235ca3001a1d55f4e6d440 100644
--- a/ash/common/system/chromeos/screen_security/screen_tray_item.cc
+++ b/ash/common/system/chromeos/screen_security/screen_tray_item.cc
@@ -101,6 +101,8 @@ void ScreenStatusView::CreateItems() {
   // TODO(bruthig): Multiline Labels don't lay out well with borders.
   // See https://crbug.com/678337 & https://crbug.com/682221.
   label_->SetBorder(nullptr);
+  TrayPopupItemStyle style(TrayPopupItemStyle::FontStyle::DEFAULT_VIEW_LABEL);
+  style.SetupLabel(label_);
 
   stop_button_ = TrayPopupUtils::CreateTrayPopupButton(this, stop_button_text_);
 }
@@ -111,19 +113,6 @@ void ScreenStatusView::UpdateFromScreenTrayItem() {
   SetVisible(screen_tray_item_->is_started());
 }
 
-void ScreenStatusView::OnNativeThemeChanged(const ui::NativeTheme* theme) {
-  if (!MaterialDesignController::IsSystemTrayMenuMaterial()) {
-    views::View::OnNativeThemeChanged(theme);
-    return;
-  }
-
-  if (theme) {
-    TrayPopupItemStyle style(theme,
-                             TrayPopupItemStyle::FontStyle::DEFAULT_VIEW_LABEL);
-    style.SetupLabel(label_);
-  }
-}
-
 ScreenNotificationDelegate::ScreenNotificationDelegate(
     ScreenTrayItem* screen_tray)
     : screen_tray_(screen_tray) {}