mash: Remove WmWindow usage from ash/system/web_notification

ash/system/status_area_widget.cc

Index: ash/system/status_area_widget.cc
diff --git a/ash/system/status_area_widget.cc b/ash/system/status_area_widget.cc
index 3f626b47954356fcd5fc12499b532bcca66aab40..b009795fc7318b86382f432b5f5e7f36f8304885 100644
--- a/ash/system/status_area_widget.cc
+++ b/ash/system/status_area_widget.cc
@@ -201,8 +201,8 @@ void StatusAreaWidget::AddSystemTray() {
 
 void StatusAreaWidget::AddWebNotificationTray() {
   DCHECK(system_tray_);
-  web_notification_tray_ = new WebNotificationTray(
-      wm_shelf_, WmWindow::Get(this->GetNativeWindow()), system_tray_);
+  web_notification_tray_ =
+      new WebNotificationTray(wm_shelf_, GetNativeWindow(), system_tray_);
   status_area_widget_delegate_->AddTray(web_notification_tray_);
 }