Reuse NativeThemeAuraDark for cros tray.

ash/common/system/status_area_widget.cc

Index: ash/common/system/status_area_widget.cc
diff --git a/ash/common/system/status_area_widget.cc b/ash/common/system/status_area_widget.cc
index 34d04a6f3e4bd30cab675388a4a5d9e3ace9e234..7ff74f3470fdf0f64e6e646fd7292529382eaf04 100644
--- a/ash/common/system/status_area_widget.cc
+++ b/ash/common/system/status_area_widget.cc
@@ -4,6 +4,7 @@
 
 #include "ash/common/system/status_area_widget.h"
 
+#include "ash/common/material_design/material_design_controller.h"
 #include "ash/common/shelf/wm_shelf.h"
 #include "ash/common/shell_window_ids.h"
 #include "ash/common/system/overview/overview_button_tray.h"
@@ -16,6 +17,7 @@
 #include "ash/common/wm_shell.h"
 #include "ash/common/wm_window.h"
 #include "base/i18n/time_formatting.h"
+#include "ui/native_theme/native_theme_dark_aura.h"
 
 #if defined(OS_CHROMEOS)
 #include "ash/common/system/chromeos/ime_menu/ime_menu_tray.h"
@@ -138,6 +140,12 @@ void StatusAreaWidget::SchedulePaint() {
   overview_button_tray_->SchedulePaint();
 }
 
+const ui::NativeTheme* StatusAreaWidget::GetNativeTheme() const {
+  return MaterialDesignController::IsShelfMaterial()
+             ? ui::NativeThemeDarkAura::instance()
+             : Widget::GetNativeTheme();
+}
+
 void StatusAreaWidget::OnNativeWidgetActivationChanged(bool active) {
   Widget::OnNativeWidgetActivationChanged(active);
   if (active)