Remove "set wallpaper..." option in system context menu when guest logged in

ash/desktop_background/desktop_background_controller.h

Index: ash/desktop_background/desktop_background_controller.h
diff --git a/ash/desktop_background/desktop_background_controller.h b/ash/desktop_background/desktop_background_controller.h
index a368443984d6c687e5dade3787aea9be2e7a525a..8d9182c47c03cca88da10fcd4f77d3c7a4830400 100644
--- a/ash/desktop_background/desktop_background_controller.h
+++ b/ash/desktop_background/desktop_background_controller.h
@@ -23,6 +23,9 @@ class UserWallpaperDelegate {
 
   // Open the set wallpaper page in the browser.
   virtual void OpenSetWallpaperPage() = 0;
+
+  // Returns true if we're logged in as a Guest.
+  virtual bool IsLoggedInAsGuest() = 0;
 };
 
 // A class to listen for login and desktop background change events and set the