GTTF: Add missing virtual destructors.

content/public/browser/download_destination_observer.h

 // Copyright (c) 2012 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 CONTENT_PUBLIC_BROWSER_DOWNLOAD_DESTINATION_OBSERVER_H_
 #define CONTENT_PUBLIC_BROWSER_DOWNLOAD_DESTINATION_OBSERVER_H_
 
 #include <string>
 
 #include "content/public/browser/download_interrupt_reasons.h"
@@ skipping 12 matching lines @@
 // Note that this interface does not deal with cross-thread lifetime issues.
 class DownloadDestinationObserver {
  public:
   virtual void DestinationUpdate(int64 bytes_so_far,
                                  int64 bytes_per_sec,
                                  const std::string& hash_state) = 0;
 
   virtual void DestinationError(DownloadInterruptReason reason) = 0;
 
   virtual void DestinationCompleted(const std::string& final_hash) = 0;
+
+ protected:
+  virtual ~DownloadDestinationObserver() { }
 };
 
 }  // namespace content
 
 #endif  // CONTENT_PUBLIC_BROWSER_DOWNLOAD_DESTINATION_OBSERVER_H_