Virtual destructors should have virtual keyword.

chrome/browser/tab_contents/thumbnail_generator.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_BROWSER_TAB_CONTENTS_THUMBNAIL_GENERATOR_H_
 #define CHROME_BROWSER_TAB_CONTENTS_THUMBNAIL_GENERATOR_H_
 #pragma once
 
 #include <map>
 #include <utility>
@@ skipping 36 matching lines @@
   // Bitmasks of options for generating a thumbnail.
   enum ThumbnailOptions {
     // No options.
     kNoOptions = 0,
     // Request a clipped thumbnail with the aspect ratio preserved.
     kClippedThumbnail = 1 << 0,
   };
 
   // This class will do nothing until you call StartThumbnailing.
   ThumbnailGenerator();
-  ~ThumbnailGenerator();
+  virtual ~ThumbnailGenerator();
 
   // Starts taking thumbnails of the given tab contents.
   void StartThumbnailing(TabContents* tab_contents);
 
   // This registers a callback that can receive the resulting SkBitmap
   // from the renderer when it is done rendering it.  This differs
   // from GetThumbnailForRenderer in that it may be asynchronous, and
   // because it will also fetch the bitmap even if the tab is hidden.
   // In addition, if the renderer has to be invoked, the scaling of
   // the thumbnail happens on the rendering thread.
@@ skipping 78 matching lines @@
   typedef std::map<int,
                    linked_ptr<AsyncRequestInfo> > ThumbnailCallbackMap;
   ThumbnailCallbackMap callback_map_;
 
   TabContents* tab_contents_;
 
   DISALLOW_COPY_AND_ASSIGN(ThumbnailGenerator);
 };
 
 #endif  // CHROME_BROWSER_TAB_CONTENTS_THUMBNAIL_GENERATOR_H_