mash: Migrate ScopedTargetRootWindow to //ash/common

ash/root_window_controller.cc

 // Copyright (c) 2012 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/root_window_controller.h"
 
 #include <queue>
 #include <vector>
 
 #include "ash/aura/aura_layout_manager_adapter.h"
@@ skipping 368 matching lines @@
   if (touch_exploration_manager_) {
     touch_exploration_manager_.reset();
   }
 #endif
 
   if (animating_wallpaper_controller_.get())
     animating_wallpaper_controller_->StopAnimating();
   wallpaper_controller_.reset();
   animating_wallpaper_controller_.reset();
   aura::Window* root_window = GetRootWindow();
-  Shell* shell = Shell::GetInstance();
+  WmWindow* root_shutting_down = WmWindowAura::Get(root_window);
+  WmShell* shell = WmShell::Get();
   // Change the target root window before closing child windows. If any child
   // being removed triggers a relayout of the shelf it will try to build a
   // window list adding windows from the target root window's containers which
   // may have already gone away.
-  if (Shell::GetTargetRootWindow() == root_window) {
-    shell->set_target_root_window(Shell::GetPrimaryRootWindow() == root_window
-                                      ? NULL
-                                      : Shell::GetPrimaryRootWindow());
+  if (shell->GetRootWindowForNewWindows() == root_shutting_down) {
+    // The root window for new windows is being destroyed. Switch to the primary
+    // root window if possible.
+    WmWindow* primary_root = shell->GetPrimaryRootWindow();
+    shell->set_root_window_for_new_windows(
+        primary_root == root_shutting_down ? nullptr : primary_root);
   }
 
   CloseChildWindows();
   GetRootWindowSettings(root_window)->controller = NULL;
   workspace_controller_.reset();
   // Forget with the display ID so that display lookup
   // ends up with invalid display.
   GetRootWindowSettings(root_window)->display_id =
       display::Display::kInvalidDisplayID;
   ash_host_->PrepareForShutdown();
@@ skipping 570 matching lines @@
     aura::Window* primary_root_window = Shell::GetInstance()
                                             ->window_tree_host_manager()
                                             ->GetPrimaryRootWindow();
     return GetRootWindowSettings(primary_root_window)->controller;
   }
 
   return GetRootWindowSettings(root_window)->controller;
 }
 
 }  // namespace ash