Forget about DesktopBackgroundWidget if it was deleted

ash/desktop_background/desktop_background_widget_controller.cc

Index: ash/desktop_background/desktop_background_widget_controller.cc
diff --git a/ash/desktop_background/desktop_background_widget_controller.cc b/ash/desktop_background/desktop_background_widget_controller.cc
index cb7a369f09120c7e68b654f28ca82e0da4610a65..8ee45ede4abefd29d07243f716c8a0f3c25482e8 100644
--- a/ash/desktop_background/desktop_background_widget_controller.cc
+++ b/ash/desktop_background/desktop_background_widget_controller.cc
@@ -34,6 +34,10 @@ DesktopBackgroundWidgetController::~DesktopBackgroundWidgetController() {
     layer_.reset(NULL);
 }
 
+void DesktopBackgroundWidgetController::CleanupWidget() {
+  widget_ = NULL;
+}
+
 void DesktopBackgroundWidgetController::SetBounds(gfx::Rect bounds) {
   if (widget_)
     widget_->SetBounds(bounds);