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

chrome/browser/chromeos/chrome_browser_main_chromeos.cc

Index: chrome/browser/chromeos/chrome_browser_main_chromeos.cc
diff --git a/chrome/browser/chromeos/chrome_browser_main_chromeos.cc b/chrome/browser/chromeos/chrome_browser_main_chromeos.cc
index 423f15f1133503fea8f9f1ae18fdf4b31ccfd610..8b2a035dd274cdb5a47953c80e1fd0f859730df2 100644
--- a/chrome/browser/chromeos/chrome_browser_main_chromeos.cc
+++ b/chrome/browser/chromeos/chrome_browser_main_chromeos.cc
@@ -36,6 +36,7 @@
 #include "chrome/browser/chromeos/login/user_manager.h"
 #include "chrome/browser/chromeos/net/cros_network_change_notifier_factory.h"
 #include "chrome/browser/chromeos/net/network_change_notifier_chromeos.h"
+#include "chrome/browser/chromeos/options2/desktop_background_observer.h"
 #include "chrome/browser/chromeos/power/brightness_observer.h"
 #include "chrome/browser/chromeos/power/resume_observer.h"
 #include "chrome/browser/chromeos/power/screen_lock_observer.h"
@@ -490,6 +491,7 @@ void ChromeBrowserMainPartsChromeos::PostBrowserStart() {
 #if defined(USE_AURA)
   // These are dependent on the ash::Shell singleton already having been
   // initialized.
+  desktop_background_observer_.reset(new chromeos::DesktopBackgroundObserver);
   power_button_observer_.reset(new chromeos::PowerButtonObserver);
   video_property_writer_.reset(new chromeos::VideoPropertyWriter);
 #endif