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 e34dba18e5683bff66c250216c9fa92049ad3957..425c130f3c49c10b7b5e8162ecedfe30ce6c291d 100644
--- a/chrome/browser/chromeos/chrome_browser_main_chromeos.cc
+++ b/chrome/browser/chromeos/chrome_browser_main_chromeos.cc
@@ -69,6 +69,10 @@
 #include <gtk/gtk.h>
 #endif
 
+#if !defined(OS_WIN) && defined(USE_ASH)
+#include "chrome/browser/chromeos/background/desktop_background_observer.h"
+#endif
+
 #if defined(USE_AURA)
 #include "chrome/browser/chromeos/legacy_window_manager/initial_browser_window_observer.h"
 #include "chrome/browser/chromeos/power/power_button_controller_delegate_chromeos.h"
@@ -409,6 +413,12 @@ void ChromeBrowserMainPartsChromeos::PreProfileInit() {
 
   // In Aura builds this will initialize ash::Shell.
   ChromeBrowserMainPartsLinux::PreProfileInit();
+
+#if !defined(OS_WIN) && defined(USE_ASH)
+  // Initialize desktop background observer so that it can receive
+  // LOGIN_USER_CHANGED notification from UserManager.
+  desktop_background_observer_.reset(new chromeos::DesktopBackgroundObserver);
+#endif
 }
 
 void ChromeBrowserMainPartsChromeos::PostProfileInit() {