Enable user change background image in settings page in Aura build.

ash/shell.cc

Index: ash/shell.cc
diff --git a/ash/shell.cc b/ash/shell.cc
index 1220f5e686d7d2774802a452dd6e979e67d6de0c..1d81bb86c39ca05c40e2067f4620a0dd91313fb5 100644
--- a/ash/shell.cc
+++ b/ash/shell.cc
@@ -8,6 +8,7 @@
 
 #include "ash/app_list/app_list.h"
 #include "ash/ash_switches.h"
+#include "ash/desktop_background/desktop_background_controller.h"
 #include "ash/drag_drop/drag_drop_controller.h"
 #include "ash/focus_cycler.h"
 #include "ash/ime/input_method_event_filter.h"
@@ -474,6 +475,7 @@ void Shell::Init() {
   AddRootWindowEventFilter(tooltip_controller_.get());
 
   drag_drop_controller_.reset(new internal::DragDropController);
+  desktop_background_controller_.reset(new DesktopBackgroundController);
   power_button_controller_.reset(new PowerButtonController);
   video_detector_.reset(new VideoDetector);
   window_cycle_controller_.reset(new WindowCycleController);