Asynchronously load wallpapers when user pod is selected.

ash/accelerators/accelerator_controller.cc

Index: ash/accelerators/accelerator_controller.cc
diff --git a/ash/accelerators/accelerator_controller.cc b/ash/accelerators/accelerator_controller.cc
index 91d771e2c444cb7ee006593a3de6ddb4ddca0e62..c3017dd7ee4a7b56cbf4a78feb6bb48d58118260 100644
--- a/ash/accelerators/accelerator_controller.cc
+++ b/ash/accelerators/accelerator_controller.cc
@@ -199,7 +199,7 @@ bool HandleToggleDesktopBackgroundMode() {
       ash::DesktopBackgroundController::BACKGROUND_IMAGE)
     desktop_background_controller->SetDesktopBackgroundSolidColorMode();
   else
-    desktop_background_controller->SetDesktopBackgroundImageMode();
+    desktop_background_controller->SetLoggedInUserWallpaper();
   return true;
 }