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

chromeos/printing/printer_discoverer.h

Index: chromeos/printing/printer_discoverer.h
diff --git a/chromeos/printing/printer_discoverer.h b/chromeos/printing/printer_discoverer.h
index 91fa715ca1de25e3b4704fc46ed3d5c24fd6aee7..6d9929accf368e91042891c0f46186174755dd92 100644
--- a/chromeos/printing/printer_discoverer.h
+++ b/chromeos/printing/printer_discoverer.h
@@ -39,6 +39,8 @@ class CHROMEOS_EXPORT PrinterDiscoverer {
   // 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;