[CUPS] Fix the occasional crash when switching between different dialogs.

chromeos/printing/fake_printer_discoverer.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 CHROMEOS_PRINTING_FAKE_PRINTER_DISCOVERER_H_
 #define CHROMEOS_PRINTING_FAKE_PRINTER_DISCOVERER_H_
 
 #include <vector>
 
 #include "base/macros.h"
 #include "base/memory/weak_ptr.h"
 #include "chromeos/printing/printer_configuration.h"
 #include "chromeos/printing/printer_discoverer.h"
 
 namespace chromeos {
 
 class FakePrinterDiscoverer : public PrinterDiscoverer {
  public:
   FakePrinterDiscoverer();
-  ~FakePrinterDiscoverer();
+  ~FakePrinterDiscoverer() override;
 
   bool StartDiscovery() override;
   bool StopDiscovery() override;
   void AddObserver(PrinterDiscoverer::Observer* observer) override;
   void RemoveObserver(PrinterDiscoverer::Observer* observer) override;
 
  private:
   void EmitPrinters(size_t start, size_t end);
 
   std::vector<chromeos::Printer> printers_;
   bool discovery_running_;
   std::vector<PrinterDiscoverer::Observer*> observers_;
 
   base::WeakPtrFactory<FakePrinterDiscoverer> weak_ptr_factory_;
 
   DISALLOW_COPY_AND_ASSIGN(FakePrinterDiscoverer);
 };
 
 }  // namespace chromeos
 
 #endif  // CHROMEOS_PRINTING_FAKE_PRINTER_DISCOVERER_H_