Make CUPS USB printing play better with the settings page. This change does several things:

chrome/browser/chromeos/printing/printers_manager.h

 // Copyright 2016 The Chromium Authors. All rights reserved.
 // Use of this source code is governed by a BSD-style license that can be
 // found in the LICENSE file.
 
 #ifndef CHROME_BROWSER_CHROMEOS_PRINTING_PRINTERS_MANAGER_H_
 #define CHROME_BROWSER_CHROMEOS_PRINTING_PRINTERS_MANAGER_H_
 
 #include <map>
 #include <memory>
 #include <string>
@@ skipping 33 matching lines @@
 
   // Register the printing preferences with the |registry|.
   static void RegisterProfilePrefs(user_prefs::PrefRegistrySyncable* registry);
 
   // Returns the printers that are saved in preferences.
   std::vector<std::unique_ptr<Printer>> GetPrinters() const;
 
   // Returns printers from enterprise policy.
   std::vector<std::unique_ptr<Printer>> GetRecommendedPrinters() const;
 
-  // Returns the printer with id |printer_id|.
+  // Returns the printer with id |printer_id|, or nullptr if no such
+  // printer exists.
   std::unique_ptr<Printer> GetPrinter(const std::string& printer_id) const;
 
   // Adds or updates a printer. Printers are identified by the id field.  Use an
   // empty id to add a new printer.
   void RegisterPrinter(std::unique_ptr<Printer> printer);
 
   // Remove printer from preferences with the id |printer_id|.  Returns true if
   // the printer was successfully removed.
   bool RemovePrinter(const std::string& printer_id);
 
@@ skipping 27 matching lines @@
       recommended_printers_;
 
   base::ObserverList<Observer> observers_;
 
   DISALLOW_COPY_AND_ASSIGN(PrintersManager);
 };
 
 }  // namespace chromeos
 
 #endif  // CHROME_BROWSER_CHROMEOS_PRINTING_PRINTERS_MANAGER_H_