Remove unused references to NativeTheme in TrayPopupItemStyle.

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

Index: ash/common/system/chromeos/screen_security/screen_tray_item.h
diff --git a/ash/common/system/chromeos/screen_security/screen_tray_item.h b/ash/common/system/chromeos/screen_security/screen_tray_item.h
index 67a531eb33cb99ce6c5237410d794a4df80ae56d..66118f76b1380a266f28d3ed2f7e0a77effdd1ac 100644
--- a/ash/common/system/chromeos/screen_security/screen_tray_item.h
+++ b/ash/common/system/chromeos/screen_security/screen_tray_item.h
@@ -57,9 +57,6 @@ class ScreenStatusView : public views::View, public views::ButtonListener {
   views::Label* label() { return label_; }
   views::Button* stop_button() { return stop_button_; }
 
-  // Overridden from views::View:
-  void OnNativeThemeChanged(const ui::NativeTheme* theme) override;
-
  private:
   // The controller for this view. May be null.
   ScreenTrayItem* screen_tray_item_;