[CUPS] Implement the Webui handler for the printers auto discovery.

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>
 
 #include "chromeos/printing/printer_configuration.h"
 
 namespace chromeos {
 
 // Interface for printer discovery.  Constructs Printer objects from USB and
 // zeroconf (DNS-SD) printers.
-class PrinterDiscoverer {
+class CHROMEOS_EXPORT PrinterDiscoverer {
  public:
   // Interface for objects interested in detected printers.
   class Observer {
    public:
     // Called after discovery has started.
     virtual void OnDiscoveryStarted() {}
 
     // Called when discovery is stopping.  OnPrintersFound will not be
     // called after this occurs.
     virtual void OnDiscoveryStopping() {}
@@ skipping 26 matching lines @@
   // 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_