Makes RootWindowController window creation in terms of common types

ash/wm/common/wm_globals.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 ASH_WM_COMMON_WM_GLOBALS_H_
 #define ASH_WM_COMMON_WM_GLOBALS_H_
 
 #include <stdint.h>
 
 #include <memory>
@@ skipping 22 matching lines @@
 // Used for accessing global state.
 class ASH_EXPORT WmGlobals {
  public:
   virtual ~WmGlobals() {}
 
   // This is necessary for a handful of places that is difficult to plumb
   // through context.
   static void Set(WmGlobals* instance);
   static WmGlobals* Get();
 
+  // Creates a new window used as a container of other windows. No painting is
+  // done to the created window.
+  virtual WmWindow* NewContainerWindow() = 0;
+
   virtual WmWindow* GetFocusedWindow() = 0;
   virtual WmWindow* GetActiveWindow() = 0;
 
   virtual WmWindow* GetPrimaryRootWindow() = 0;
 
   // Returns the root window for the specified display.
   virtual WmWindow* GetRootWindowForDisplayId(int64_t display_id) = 0;
 
   // Returns the root window that newly created windows should be added to.
   // NOTE: this returns the root, newly created window should be added to the
@@ skipping 44 matching lines @@
   virtual void RemoveOverviewModeObserver(WmOverviewModeObserver* observer) = 0;
 
  private:
   static WmGlobals* instance_;
 };
 
 }  // namespace wm
 }  // namespace ash
 
 #endif  // ASH_WM_COMMON_WM_GLOBALS_H_