chromeos: Remove some WmWindow usage from ash/system

ash/system/tray/system_tray.cc

Index: ash/system/tray/system_tray.cc
diff --git a/ash/system/tray/system_tray.cc b/ash/system/tray/system_tray.cc
index 49c382b7d5d7ad11319dc3f92d62625e971aaad0..a6d65bc2bb915b552a9d8df9af72c7998a295cf5 100644
--- a/ash/system/tray/system_tray.cc
+++ b/ash/system/tray/system_tray.cc
@@ -50,7 +50,6 @@
 #include "ash/system/web_notification/web_notification_tray.h"
 #include "ash/wm/container_finder.h"
 #include "ash/wm/widget_finder.h"
-#include "ash/wm_window.h"
 #include "base/logging.h"
 #include "base/memory/ptr_util.h"
 #include "base/metrics/histogram_macros.h"
@@ -599,8 +598,7 @@ void SystemTray::OnBeforeBubbleWidgetInit(
     views::Widget* bubble_widget,
     views::Widget::InitParams* params) const {
   // Place the bubble in the same root window as |anchor_widget|.
-  WmWindow::Get(anchor_widget->GetNativeWindow())
-      ->GetRootWindowController()
+  RootWindowController::ForWindow(anchor_widget->GetNativeWindow())
       ->ConfigureWidgetInitParamsForContainer(
           bubble_widget, kShellWindowId_SettingBubbleContainer, params);
 }