Move the logic of getting root window to StartObserving()

ash/system/web_notification/ash_popup_alignment_delegate.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/system/web_notification/ash_popup_alignment_delegate.h"
 
 #include "ash/display/display_controller.h"
 #include "ash/shelf/shelf_constants.h"
 #include "ash/shelf/shelf_layout_manager.h"
 #include "ash/shelf/shelf_types.h"
@@ skipping 33 matching lines @@
     screen_->RemoveObserver(this);
   Shell::GetInstance()->RemoveShellObserver(this);
   if (shelf_)
     shelf_->RemoveObserver(this);
 }
 
 void AshPopupAlignmentDelegate::StartObserving(gfx::Screen* screen,
                                                const gfx::Display& display) {
   screen_ = screen;
   display_id_ = display.id();
+  root_window_ = ash::Shell::GetInstance()->display_controller()->
+      GetRootWindowForDisplayId(display_id_);
   UpdateShelf();
   screen->AddObserver(this);
   Shell::GetInstance()->AddShellObserver(this);
   if (system_tray_height_ > 0)
     OnAutoHideStateChanged(shelf_->auto_hide_state());
 }
 
 void AshPopupAlignmentDelegate::SetSystemTrayHeight(int height) {
   system_tray_height_ = height;
 
@@ skipping 53 matching lines @@
 }
 
 ShelfAlignment AshPopupAlignmentDelegate::GetAlignment() const {
   return shelf_ ? shelf_->GetAlignment() : SHELF_ALIGNMENT_BOTTOM;
 }
 
 void AshPopupAlignmentDelegate::UpdateShelf() {
   if (shelf_)
     return;
 
-  aura::Window* root_window = ash::Shell::GetInstance()->display_controller()->
-      GetRootWindowForDisplayId(display_id_);
-  shelf_ = ShelfLayoutManager::ForShelf(root_window);
+  shelf_ = ShelfLayoutManager::ForShelf(root_window_);
   if (shelf_)
     shelf_->AddObserver(this);
 }
 
 void AshPopupAlignmentDelegate::OnDisplayWorkAreaInsetsChanged() {
   UpdateShelf();
 
   work_area_ = Shell::GetScreen()->GetDisplayNearestWindow(
       shelf_->shelf_widget()->GetNativeView()).work_area();
 }
@@ skipping 27 matching lines @@
 }
 
 void AshPopupAlignmentDelegate::OnDisplayMetricsChanged(
     const gfx::Display& display,
     uint32_t metrics) {
   if (display.id() == display_id_ && shelf_)
     OnAutoHideStateChanged(shelf_->auto_hide_state());
 }
 
 }  // namespace ash