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

chrome/browser/chromeos/chrome_browser_main_chromeos.h

Index: chrome/browser/chromeos/chrome_browser_main_chromeos.h
diff --git a/chrome/browser/chromeos/chrome_browser_main_chromeos.h b/chrome/browser/chromeos/chrome_browser_main_chromeos.h
index 156dc58b20070bcbaa2331f19d60d242e948a1b1..e00a0188c7801639669e8207f1928b7bae23efae 100644
--- a/chrome/browser/chromeos/chrome_browser_main_chromeos.h
+++ b/chrome/browser/chromeos/chrome_browser_main_chromeos.h
@@ -15,6 +15,7 @@ class ScreenLockObserver;
 class SessionManagerObserver;
 
 #if defined(USE_AURA)
+class DesktopBackgroundObserver;
 class InitialBrowserWindowObserver;
 class PowerButtonObserver;
 class VideoPropertyWriter;
@@ -48,6 +49,7 @@ class ChromeBrowserMainPartsChromeos : public ChromeBrowserMainPartsLinux {
   scoped_ptr<chromeos::SessionManagerObserver> session_manager_observer_;
 
 #if defined(USE_AURA)
+  scoped_ptr<chromeos::DesktopBackgroundObserver> desktop_background_observer_;
   scoped_ptr<chromeos::InitialBrowserWindowObserver>
       initial_browser_window_observer_;
   scoped_ptr<chromeos::PowerButtonObserver> power_button_observer_;