Asynchronously load wallpapers when user pod is selected.

ash/shell.cc

Index: ash/shell.cc
diff --git a/ash/shell.cc b/ash/shell.cc
index bc8829a1fa9de4936c7361af2d090eb71d965a84..19944f273fc74bf39ee143cf55b2d6fa66c9d9de 100644
--- a/ash/shell.cc
+++ b/ash/shell.cc
@@ -230,7 +230,7 @@ class DummyUserWallpaperDelegate : public UserWallpaperDelegate {
   virtual ~DummyUserWallpaperDelegate() {}
 
   virtual const int GetUserWallpaperIndex() OVERRIDE {
-    return 0;
+    return -1;
   }
 
   virtual void OpenSetWallpaperPage() OVERRIDE {
@@ -723,7 +723,7 @@ void Shell::Init() {
 
   // It needs to be created after OnWindowResized has been called, otherwise the
   // widget will not paint when restoring after a browser crash.
-  desktop_background_controller_->SetDesktopBackgroundImageMode();
+  desktop_background_controller_->SetLoggedInUserWallpaper();
 
   window_modality_controller_.reset(new internal::WindowModalityController);
   AddRootWindowEventFilter(window_modality_controller_.get());