Desktop aura: Break aura::Window::SetParent in two.

ui/views/widget/widget.h

 // 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.
 
 #ifndef UI_VIEWS_WIDGET_WIDGET_H_
 #define UI_VIEWS_WIDGET_WIDGET_H_
 
 #include <set>
 #include <stack>
 #include <vector>
@@ skipping 175 matching lines @@
     // of the default one.
     // TODO(beng): Figure out if there's a better way to expose this, e.g. get
     // rid of NW subclasses and do this all via message handling.
     DesktopRootWindowHost* desktop_root_window_host;
     bool top_level;
     // Only used by NativeWidgetAura. Specifies the type of layer for the
     // aura::Window. Default is LAYER_TEXTURED.
     ui::LayerType layer_type;
     // Only used by Aura. Provides additional context (generally a RootWindow)
     // during creation to allow the widget to determine which desktop type it
-    // will belong to. NULL indicates a normal native desktop.
+    // will belong to. NULL is not allowed if you are using aura.
     gfx::NativeView context;
   };
 
   Widget();
   virtual ~Widget();
 
   // Creates a decorated window Widget with the specified properties.
   static Widget* CreateWindow(WidgetDelegate* delegate);
   static Widget* CreateWindowWithParent(WidgetDelegate* delegate,
                                         gfx::NativeWindow parent);
@@ skipping 604 matching lines @@
 
   // Is |root_layers_| out of date?
   bool root_layers_dirty_;
 
   DISALLOW_COPY_AND_ASSIGN(Widget);
 };
 
 }  // namespace views
 
 #endif  // UI_VIEWS_WIDGET_WIDGET_H_