Makes RootWindowController window creation in terms of common types

mash/wm/bridge/wm_globals_mus.cc

 // Copyright 2016 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 "mash/wm/bridge/wm_globals_mus.h"
 
 #include "ash/wm/common/window_resizer.h"
 #include "ash/wm/common/wm_activation_observer.h"
 #include "ash/wm/common/wm_shell_window_ids.h"
 #include "base/memory/ptr_util.h"
@@ skipping 53 matching lines @@
     int64_t id) {
   for (WmRootWindowControllerMus* root_window_controller :
        root_window_controllers_) {
     if (root_window_controller->GetDisplay().id() == id)
       return root_window_controller;
   }
   NOTREACHED();
   return nullptr;
 }
 
+ash::wm::WmWindow* WmGlobalsMus::NewContainerWindow() {
+  return WmWindowMus::Get(client_->NewWindow());
+}
+
 ash::wm::WmWindow* WmGlobalsMus::GetFocusedWindow() {
   return WmWindowMus::Get(client_->GetFocusedWindow());
 }
 
 ash::wm::WmWindow* WmGlobalsMus::GetActiveWindow() {
   return GetToplevelAncestor(client_->GetFocusedWindow());
 }
 
 ash::wm::WmWindow* WmGlobalsMus::GetPrimaryRootWindow() {
   return root_window_controllers_[0]->GetWindow();
@@ skipping 131 matching lines @@
 }
 
 void WmGlobalsMus::OnWillDestroyClient(
     mus::WindowTreeClient* client) {
   DCHECK_EQ(client, client_);
   RemoveClientObserver();
 }
 
 }  // namespace wm
 }  // namespace mash