Makes RootWindowController window creation in terms of common types

mash/wm/bridge/wm_globals_mus.h

 // 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.
 
 #ifndef MASH_WM_BRIDGE_WM_GLOBALS_MUS_H_
 #define MASH_WM_BRIDGE_WM_GLOBALS_MUS_H_
 
 #include <stdint.h>
 
 #include <vector>
@@ skipping 25 matching lines @@
   void AddRootWindowController(WmRootWindowControllerMus* controller);
   void RemoveRootWindowController(WmRootWindowControllerMus* controller);
 
   // Returns the ancestor of |window| (including |window|) that is considered
   // toplevel. |window| may be null.
   static WmWindowMus* GetToplevelAncestor(mus::Window* window);
 
   WmRootWindowControllerMus* GetRootWindowControllerWithDisplayId(int64_t id);
 
   // WmGlobals:
+  ash::wm::WmWindow* NewContainerWindow() override;
   ash::wm::WmWindow* GetFocusedWindow() override;
   ash::wm::WmWindow* GetActiveWindow() override;
   ash::wm::WmWindow* GetPrimaryRootWindow() override;
   ash::wm::WmWindow* GetRootWindowForDisplayId(int64_t display_id) override;
   ash::wm::WmWindow* GetRootWindowForNewWindows() override;
   std::vector<ash::wm::WmWindow*> GetMruWindowList() override;
   std::vector<ash::wm::WmWindow*> GetMruWindowListIgnoreModals() override;
   bool IsForceMaximizeOnFirstRun() override;
   bool IsUserSessionBlocked() override;
   bool IsScreenLocked() override;
@@ skipping 33 matching lines @@
 
   base::ObserverList<ash::wm::WmActivationObserver> activation_observers_;
 
   DISALLOW_COPY_AND_ASSIGN(WmGlobalsMus);
 };
 
 }  // namespace wm
 }  // namespace mash
 
 #endif  // MASH_WM_BRIDGE_WM_GLOBALS_MUS_H_