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

chrome/browser/chromeos/printing/printers_manager.h

Index: chrome/browser/chromeos/printing/printers_manager.h
diff --git a/chrome/browser/chromeos/printing/printers_manager.h b/chrome/browser/chromeos/printing/printers_manager.h
index 04eb0dda72a1e42fd2d3d57517a3cc595066fdf1..f31bd6c7d0fd766ea22b6dbfa04392dd6a22b574 100644
--- a/chrome/browser/chromeos/printing/printers_manager.h
+++ b/chrome/browser/chromeos/printing/printers_manager.h
@@ -51,7 +51,8 @@ class PrintersManager : public KeyedService {
   // 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