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

ash/desktop_background/desktop_background_controller.cc

 // 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.
 
 #include "ash/desktop_background/desktop_background_controller.h"
 
 #include "ash/desktop_background/desktop_background_view.h"
 #include "ash/shell.h"
 #include "ash/shell_factory.h"
 #include "ash/shell_window_ids.h"
@@ skipping 107 matching lines @@
   return GetWallpaper();
 }
 
 void DesktopBackgroundController::OnRootWindowAdded(
     aura::RootWindow* root_window) {
   switch (desktop_background_mode_) {
     case BACKGROUND_IMAGE:
       if (current_wallpaper_.get()) {
         SetDesktopBackgroundImage(root_window);
       } else {
-        internal::CreateDesktopBackground(root_window);
+        internal::CreateDesktopBackground(root_window,
+            ash::internal::kShellWindowId_DesktopBackgroundContainer);
       }
       break;
     case BACKGROUND_SOLID_COLOR:
       SetDesktopBackgroundSolidColorMode(background_color_);
       break;
   }
 }
 
 void DesktopBackgroundController::SetDefaultWallpaper(int index) {
   // We should not change background when index is invalid. For instance, at
@@ skipping 44 matching lines @@
   // Set a solid black background.
   // TODO(derat): Remove this in favor of having the compositor only clear the
   // viewport when there are regions not covered by a layer:
   // http://crbug.com/113445
   current_wallpaper_.reset(NULL);
   background_color_ = color;
   desktop_background_mode_ = BACKGROUND_SOLID_COLOR;
   Shell::RootWindowList root_windows = Shell::GetAllRootWindows();
   for (Shell::RootWindowList::iterator iter = root_windows.begin();
        iter != root_windows.end(); ++iter) {
+    aura::RootWindow* root_window = *iter;
+
+    SetColorLayerForContainer(color, root_window,
+        internal::kShellWindowId_DesktopBackgroundContainer);
+  }
+}
+
+void DesktopBackgroundController::SetColorLayerForContainer(
+    SkColor color,
+    aura::RootWindow* root_window,
+    int container_id) {
+

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

nit: remove newline.

[Denis Kuznetsov (DE-MUC), 2012/07/23 14:01:08]

Done.

     ui::Layer* background_layer = new ui::Layer(ui::LAYER_SOLID_COLOR);
     background_layer->SetColor(color);
-    aura::RootWindow* root_window = *iter;
-    Shell::GetContainer(
-        root_window,
-        internal::kShellWindowId_DesktopBackgroundContainer)->
+
+    Shell::GetContainer(root_window,container_id)->
         layer()->Add(background_layer);
     GetRootWindowLayoutManager(root_window)->SetBackgroundLayer(
-        background_layer);
-    GetRootWindowLayoutManager(root_window)->SetBackgroundWidget(NULL);
-  }
+        background_layer,
+        container_id);
+    GetRootWindowLayoutManager(root_window)->SetBackgroundWidget(
+        NULL,
+        container_id);
 }
 
 void DesktopBackgroundController::SetDesktopBackgroundImage(
     aura::RootWindow* root_window) {
-  GetRootWindowLayoutManager(root_window)->SetBackgroundLayer(NULL);
+  GetRootWindowLayoutManager(root_window)->SetBackgroundLayer(NULL,
+      internal::kShellWindowId_DesktopBackgroundContainer);
   if (current_wallpaper_.get() &&
       !current_wallpaper_->wallpaper_image.empty())
-    internal::CreateDesktopBackground(root_window);
+    internal::CreateDesktopBackground(root_window,
+        ash::internal::kShellWindowId_DesktopBackgroundContainer);
 }
 
 void DesktopBackgroundController::UpdateDesktopBackgroundImageMode() {
   Shell::RootWindowList root_windows = Shell::GetAllRootWindows();
 
   for (Shell::RootWindowList::iterator iter = root_windows.begin();
        iter != root_windows.end(); ++iter) {
     SetDesktopBackgroundImage(*iter);
   }
   desktop_background_mode_ = BACKGROUND_IMAGE;
 }
 
 void DesktopBackgroundController::OnWallpaperLoadCompleted(
     scoped_refptr<WallpaperOperation> wo) {
   current_wallpaper_.reset(wo->ReleaseWallpaperData());
 
   UpdateDesktopBackgroundImageMode();
 
   DCHECK(wo.get() == wallpaper_op_.get());
   wallpaper_op_ = NULL;
 }
 
 void DesktopBackgroundController::CreateEmptyWallpaper() {
   current_wallpaper_.reset(NULL);
   desktop_background_mode_ = BACKGROUND_IMAGE;
   Shell::RootWindowList root_windows = Shell::GetAllRootWindows();
   for (Shell::RootWindowList::iterator iter = root_windows.begin();
        iter != root_windows.end(); ++iter) {
-    internal::CreateDesktopBackground(*iter);
+    internal::CreateDesktopBackground(*iter,
+        ash::internal::kShellWindowId_DesktopBackgroundContainer);
   }
 }
 
+void DesktopBackgroundController::CreateLockScreenDesktops() {

[bshe, 2012/07/23 00:19:36]

I am guessing you plan use the new container to replace the data source solution
you previously implement for wallpaper at lock screen? In that case, do we still
need the post processed wallpaper? It seems the new lock screen container will
paint exactly the way DesktopBackgroundContainer did.

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

Yes, this is the plan. However, there is an issue with graphics acceleration
(that is required for transparency in webUI), that costs us about 700ms on
startup. So we leave data source solution in code as a backup plan.
So that task is lower priority now, but it can not be dropped yet.

[bshe, 2012/07/23 14:36:28]

Got it. I will drop the priority of post processed wallpaper for now. Actually,
it seems there is no reliable way to get a screen size bitmap from desktop
background view. I originally thought I can use the canvas object in OnPaint
function to extract the bitmap. But it seems we can't actually rely on canvas
since it may only draw a tile of the bitmap at one time. So we probably need
some ugly hacks to get the screen size bitmap from desktop background view. I
will only work on it if it becomes real necessary. 

On 2012/07/23 14:01:09, Denis Kuznetsov wrote:

+  Shell::RootWindowList root_windows = Shell::GetAllRootWindows();
+  for (Shell::RootWindowList::iterator iter = root_windows.begin();
+       iter != root_windows.end(); ++iter) {
+       if (BACKGROUND_IMAGE == desktop_background_mode_) {

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

spacing is off in the rest of this function.

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

Done.

+         internal::CreateDesktopBackground(*iter,
+             ash::internal::kShellWindowId_LockScreenBackgroundContainer);
+       } else if (BACKGROUND_SOLID_COLOR == desktop_background_mode_) {
+         SetColorLayerForContainer(background_color_, *iter,
+             ash::internal::kShellWindowId_LockScreenBackgroundContainer);
+       } else {
+       // If something went wrong, still use some default color to
+       // hide user's windows.
+         SetColorLayerForContainer(SK_ColorGRAY, *iter,
+             ash::internal::kShellWindowId_LockScreenBackgroundContainer);
+       }
+  }
+}
+
+void DesktopBackgroundController::RemoveLockScreenDesktops() {
+Shell::RootWindowList root_windows = Shell::GetAllRootWindows();

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

spacing is off.

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

Done.

+   for (Shell::RootWindowList::iterator iter = root_windows.begin();
+       iter != root_windows.end(); ++iter) {
+       GetRootWindowLayoutManager(*iter)->DropBackgroundForContainer(
+           ash::internal::kShellWindowId_LockScreenBackgroundContainer);
+  }
+}
+
+
 }  // namespace ash