2nd display should show the same background as login/lock screen:

ash/wm/root_window_layout_manager.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 ASH_WM_ROOT_WINDOW_LAYOUT_MANAGER_H_
 #define ASH_WM_ROOT_WINDOW_LAYOUT_MANAGER_H_
 
+#include "ash/shell_window_ids.h"
 #include "base/basictypes.h"
 #include "base/compiler_specific.h"
 #include "base/memory/scoped_ptr.h"
 #include "ui/aura/layout_manager.h"
 
 namespace aura {
 class Window;
 }
 namespace gfx {
 class Rect;
 }
 namespace ui {
 class Layer;
 }
 namespace views {
 class Widget;
 }
 
 namespace ash {
 namespace internal {
 
 // A layout manager for the root window.
 // Resizes all of its immediate children to fill the bounds of the root window.
 class RootWindowLayoutManager : public aura::LayoutManager {
  public:
   explicit RootWindowLayoutManager(aura::Window* owner);
   virtual ~RootWindowLayoutManager();
 
-  views::Widget* background_widget() { return background_widget_; }
-  ui::Layer* background_layer() { return background_layer_.get(); }
+  views::Widget* background_widget() {
+      return background_widget_[kShellWindowId_DesktopBackgroundContainer];
+  }
+  ui::Layer* background_layer() {
+      return background_layer_[kShellWindowId_DesktopBackgroundContainer].get();
+  }
 
-  // Sets the background to |widget|. Closes and destroys the old widget if it
-  // exists and differs from the new widget.
-  void SetBackgroundWidget(views::Widget* widget);
+  // Sets the background in container |container_id| to |widget|.
+  // Closes and destroys the old widget for that container if it exists and
+  // differs from the new widget.
+  void SetBackgroundWidget(views::Widget* widget, int container_id);
 
-  // Sets a background layer, taking ownership of |layer|.  This is provided as
-  // a lightweight alternative to SetBackgroundWidget(); layers can be simple
-  // colored quads instead of being textured.
-  void SetBackgroundLayer(ui::Layer* layer);
+  // Sets a background layer for container |container_id|, taking ownership of
+  // |layer|.  This is provided as a lightweight alternative to
+  // SetBackgroundWidget(); layers can be simple colored quads instead of being
+  // textured.
+  void SetBackgroundLayer(ui::Layer* layer, int container_id);
+
+  void DropBackgroundForContainer(int container_id);
 
   // Overridden from aura::LayoutManager:
   virtual void OnWindowResized() OVERRIDE;
   virtual void OnWindowAddedToLayout(aura::Window* child) OVERRIDE;
   virtual void OnWillRemoveWindowFromLayout(aura::Window* child) OVERRIDE;
   virtual void OnWindowRemovedFromLayout(aura::Window* child) OVERRIDE;
   virtual void OnChildWindowVisibilityChanged(aura::Window* child,
                                               bool visible) OVERRIDE;
   virtual void SetChildBounds(aura::Window* child,
                               const gfx::Rect& requested_bounds) OVERRIDE;
 
  private:
   aura::Window* owner_;
 
-  // May be NULL if we're not painting a background.
-  views::Widget* background_widget_;
-  scoped_ptr<ui::Layer> background_layer_;
+  // May be NULL if we're not painting a background for that container.
+  views::Widget* background_widget_[kShellWindowCount];

[sky, 2012/07/20 23:57:04]

I don't understand this. Why do we need a background widget/layer per container?

[Denis Kuznetsov (DE-MUC), 2012/07/23 14:01:09]

I'd say we have a kind of technical debt here: it is actually a layout manager,
and it should not know anything about Background - only about the components to
lay off.
However, existing code names these components as "Background", have just one of
each type, and handles lifetime management for them (using scoped_ptr /
CloseNow()).

On my side of changes, I need to add two more possible objects (Widget/Layer) to
existing code. I can do it by either using named fields (and introducing new set
methods), or by adding some identifier parameter to existing methods.
I already have "natural" identifier - id of container. I could introduce some
another id, and keep mapping at some place in code, but I'm not sure it would
make code more clear (although we have about 200 bytes overhead with this
solution).

I can rename existing methods to remove confusing "background" name from them
(e.g. replace it with "FullScreen"/"FullWindow"), and make memory management a
bit more clear in code.

I already have natural ID 

+  scoped_ptr<ui::Layer> background_layer_[kShellWindowCount];
 
   DISALLOW_COPY_AND_ASSIGN(RootWindowLayoutManager);
 };
 
 }  // namespace internal
 }  // namespace ash
 
 #endif  // ASH_WM_ROOT_WINDOW_LAYOUT_MANAGER_H_