Add a container window to host the desktop window hierarchy in AURA.

ui/views/widget/desktop_aura/desktop_focus_rules.h

Index: ui/views/widget/desktop_aura/desktop_focus_rules.h
===================================================================
--- ui/views/widget/desktop_aura/desktop_focus_rules.h	(revision 224198)
+++ ui/views/widget/desktop_aura/desktop_focus_rules.h	(working copy)
@@ -11,13 +11,17 @@
 
 class DesktopFocusRules : public corewm::BaseFocusRules {
  public:
-  DesktopFocusRules();
+  explicit DesktopFocusRules(aura::Window* content_window);
   virtual ~DesktopFocusRules();
 
  private:
   // Overridden from corewm::BaseFocusRules:
   virtual bool SupportsChildActivation(aura::Window* window) const OVERRIDE;
 
+  // The content window. This is an activatable window even though it is a
+  // child.
+  aura::Window* content_window_;
+
   DISALLOW_COPY_AND_ASSIGN(DesktopFocusRules);
 };