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

chromeos/printing/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_PRINTER_DISCOVERER_H_
 #define CHROMEOS_PRINTING_PRINTER_DISCOVERER_H_
 
 #include <memory>
 #include <vector>
 
@@ skipping 21 matching lines @@
 
     // Called with a collection of printers as they are discovered.  Printer
     // objects must be copied if they are retained outside of the scope of this
     // function.
     virtual void OnPrintersFound(const std::vector<Printer>& printers) {}
   };
 
   // Static factory
   static std::unique_ptr<PrinterDiscoverer> Create();
 
+  virtual ~PrinterDiscoverer() {}
+
   // Begin scanning for printers.  Found printers will be reported to the
   // attached observer.
   virtual bool StartDiscovery() = 0;
 
   // Stop scanning for printers.  Returns false if scanning could not be stopped
   // and new results will continue to be be sent to observers.  Returns true if
   // scanning was stopped successfully.  No calls to the attached observer will
   // be made after this returns if it returned true.
   virtual bool StopDiscovery() = 0;
 
   // Add an observer that will be notified of discovered printers.  Ownership of
   // |observer| is not taken by the discoverer.  It is an error to add an
   // oberserver more than once.
   virtual void AddObserver(PrinterDiscoverer::Observer* observer) = 0;
 
   // Remove an observer of printer discovery.
   virtual void RemoveObserver(PrinterDiscoverer::Observer* observer) = 0;
 };
 
 }  // namespace chromeos
 
 #endif  // CHROMEOS_PRINTING_PRINTER_DISCOVERER_H_