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 4bf3725d195544f6153bcf1632ee51cf2532f134..9c61c1654aa5d26b6d59d6f20d0861aa4fcdbe56 100644
--- a/chrome/browser/chromeos/chrome_browser_main_chromeos.cc
+++ b/chrome/browser/chromeos/chrome_browser_main_chromeos.cc
@@ -70,6 +70,10 @@
 #include <gtk/gtk.h>
 #endif
 
+#if 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"
@@ -370,6 +374,12 @@ void ChromeBrowserMainPartsChromeos::PreProfileInit() {
     chromeos::ScreenLocker::InitClass();
   }
 
+#if 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
+
   // This forces the ProfileManager to be created and register for the
   // notification it needs to track the logged in user.
   g_browser_process->profile_manager();