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

ash/shell_factory.h

Index: ash/shell_factory.h
diff --git a/ash/shell_factory.h b/ash/shell_factory.h
index 0faec35ae09b25f0adc06670a686e6b13d509aec..b90889bcf185c67b4586a1adb1fb795dfe9e9c80 100644
--- a/ash/shell_factory.h
+++ b/ash/shell_factory.h
@@ -8,6 +8,8 @@
 
 #include "ash/ash_export.h"
 
+class SkBitmap;
+
 namespace views {
 class View;
 class Widget;
@@ -18,7 +20,7 @@ class Widget;
 namespace ash {
 
 namespace internal {
-views::Widget* CreateDesktopBackground();
+views::Widget* CreateDesktopBackground(const SkBitmap& wallpaper);
 ASH_EXPORT views::Widget* CreateStatusArea(views::View* contents);
 }  // namespace internal