Fix leak of callback object in r86518.

chrome/browser/download/download_history.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_DOWNLOAD_DOWNLOAD_HISTORY_H_
 #define CHROME_BROWSER_DOWNLOAD_DOWNLOAD_HISTORY_H_
 #pragma once
 
 #include <map>
 
@@ skipping 16 matching lines @@
   // A fake download table ID which represents a download that has started,
   // but is not yet in the table.
   static const int kUninitializedHandle;
 
   explicit DownloadHistory(Profile* profile);
   ~DownloadHistory();
 
   // Retrieves DownloadCreateInfos saved in the history.
   void Load(HistoryService::DownloadQueryCallback* callback);
 
-  // Checks whether |referrer_url| has been visited before today.
+  // Checks whether |referrer_url| has been visited before today.  This takes
+  // ownership of |callback|.
   void CheckVisitedReferrerBefore(int32 download_id,
                                   const GURL& referrer_url,
                                   VisitedBeforeDoneCallback* callback);
 
   // Adds a new entry for a download to the history database.
   void AddEntry(DownloadItem* download_item,
                 HistoryService::DownloadCreateCallback* callback);
 
   // Updates the history entry for |download_item|.
   void UpdateEntry(DownloadItem* download_item);
@@ skipping 32 matching lines @@
 
   CancelableRequestConsumer history_consumer_;
 
   // The outstanding requests made by CheckVisitedReferrerBefore().
   VisitedBeforeRequestsMap visited_before_requests_;
 
   DISALLOW_COPY_AND_ASSIGN(DownloadHistory);
 };
 
 #endif  // CHROME_BROWSER_DOWNLOAD_DOWNLOAD_HISTORY_H_