Fixed crash in the proxy. Reference has been passed between threads,

chrome/service/cloud_print/cloud_print_proxy_backend.h

 // Copyright (c) 2011 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 CHROME_SERVICE_CLOUD_PRINT_CLOUD_PRINT_PROXY_BACKEND_H_
 #define CHROME_SERVICE_CLOUD_PRINT_CLOUD_PRINT_PROXY_BACKEND_H_
 #pragma once
 
 #include <string>
 
@@ skipping 13 matching lines @@
 
 // CloudPrintProxyFrontend is the interface used by CloudPrintProxyBackend to
 // communicate with the entity that created it and, presumably, is interested in
 // cloud print proxy related activity.
 // NOTE: All methods will be invoked by a CloudPrintProxyBackend on the same
 // thread used to create that CloudPrintProxyBackend.
 class CloudPrintProxyFrontend {
  public:
   CloudPrintProxyFrontend() {}
 
-  // There is a list of printers available that can be registered.
-  virtual void OnPrinterListAvailable(
-      const printing::PrinterList& printer_list) = 0;
   // We successfully authenticated with the cloud print server. This callback
   // allows the frontend to persist the tokens.
   virtual void OnAuthenticated(const std::string& robot_oauth_refresh_token,
                                const std::string& robot_email,
                                const std::string& user_email) = 0;
   // We have invalid/expired credentials.
   virtual void OnAuthenticationFailed() = 0;
   // The print system could not be initialized.
   virtual void OnPrintSystemUnavailable() = 0;
 
@@ skipping 56 matching lines @@
   MessageLoop* const frontend_loop_;
   // The frontend which is responsible for displaying UI and updating Prefs
   CloudPrintProxyFrontend* frontend_;
 
   friend class base::RefCountedThreadSafe<CloudPrintProxyBackend::Core>;
 
   DISALLOW_COPY_AND_ASSIGN(CloudPrintProxyBackend);
 };
 
 #endif  // CHROME_SERVICE_CLOUD_PRINT_CLOUD_PRINT_PROXY_BACKEND_H_