ash: Move ShelfWidget ownership to WmShelf and refactor access to it

ash/test/status_area_widget_test_helper.cc

 // Copyright 2014 The Chromium Authors. All rights reserved.
 // Use of this source code is governed by a BSD-style license that can be
 // found in the LICENSE file.
 
 #include "ash/test/status_area_widget_test_helper.h"
 
-#include "ash/common/shelf/shelf_widget.h"
 #include "ash/common/system/status_area_widget.h"
 #include "ash/common/system/tray/system_tray_delegate.h"
 #include "ash/common/wm_shell.h"
 #include "ash/root_window_controller.h"
 #include "ash/shell.h"
 
 namespace ash {
 
 LoginStatus StatusAreaWidgetTestHelper::GetUserLoginStatus() {
   return WmShell::Get()->system_tray_delegate()->GetUserLoginStatus();
 }
 
 StatusAreaWidget* StatusAreaWidgetTestHelper::GetStatusAreaWidget() {
-  return Shell::GetPrimaryRootWindowController()
-      ->shelf_widget()
-      ->status_area_widget();
+  return Shell::GetPrimaryRootWindowController()->GetStatusAreaWidget();
 }
 
 StatusAreaWidget* StatusAreaWidgetTestHelper::GetSecondaryStatusAreaWidget() {
   RootWindowController* primary_controller =
       Shell::GetPrimaryRootWindowController();
   Shell::RootWindowControllerList controllers =
       Shell::GetAllRootWindowControllers();
   for (size_t i = 0; i < controllers.size(); ++i) {
     if (controllers[i] != primary_controller)
-      return controllers[i]->shelf_widget()->status_area_widget();
+      return controllers[i]->GetStatusAreaWidget();
   }
 
-  return NULL;
+  return nullptr;
 }
 
 }  // namespace ash