Makes RootWindowController window creation in terms of common types

ash/wm/common/wm_globals.h

Index: ash/wm/common/wm_globals.h
diff --git a/ash/wm/common/wm_globals.h b/ash/wm/common/wm_globals.h
index 7549cd35519fbcb078870c366f5025245be7e777..a57f308fa0d3b562d15decb95904052bfae5d2d3 100644
--- a/ash/wm/common/wm_globals.h
+++ b/ash/wm/common/wm_globals.h
@@ -40,6 +40,10 @@ class ASH_EXPORT WmGlobals {
   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;