Virtual destructors should have virtual keyword.

chrome/browser/history/in_memory_history_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.
 
 // Contains the history backend wrapper around the in-memory URL database. This
 // object maintains an in-memory cache of the subset of history required to do
 // in-line autocomplete.
 //
 // It is created on the history thread and passed to the main thread where
 // operations can be completed synchronously. It listens for notifications
@@ skipping 21 matching lines @@
 class InMemoryDatabase;
 class InMemoryURLIndex;
 struct KeywordSearchTermDetails;
 class URLDatabase;
 struct URLsDeletedDetails;
 struct URLsModifiedDetails;
 
 class InMemoryHistoryBackend : public NotificationObserver {
  public:
   InMemoryHistoryBackend();
-  ~InMemoryHistoryBackend();
+  virtual ~InMemoryHistoryBackend();
 
   // Initializes the backend from the history database pointed to by the
   // full path in |history_filename|. |history_dir| is the path to the
   // directory containing the history database and is also used
   // as the directory where the InMemoryURLIndex's cache is kept. |db| is
   // used for building the InMemoryURLIndex. |languages| gives the
   // preferred user languages with which URLs and page titles are
   // interpreted while decomposing into words and characters during indexing.
   bool Init(const FilePath& history_filename,
             const FilePath& history_dir,
@@ skipping 45 matching lines @@
 
   // The index used for quick history lookups.
   scoped_ptr<history::InMemoryURLIndex> index_;
 
   DISALLOW_COPY_AND_ASSIGN(InMemoryHistoryBackend);
 };
 
 }  // namespace history
 
 #endif  // CHROME_BROWSER_HISTORY_IN_MEMORY_HISTORY_BACKEND_H_