Make DownloadItem derive SupportsUserData instead of re-implementing it.

chrome/browser/download/download_completion_blocker.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 CHROME_BROWSER_DOWNLOAD_DOWNLOAD_COMPLETION_BLOCKER_H_
 #define CHROME_BROWSER_DOWNLOAD_DOWNLOAD_COMPLETION_BLOCKER_H_
 
 #include "base/callback.h"
 #include "content/public/browser/download_item.h"
 
 // A subsystem may use a DownloadCompletionBlocker in conjunction with
 // DownloadManagerDelegate::ShouldCompleteDownload() in order to block the
 // completion of a DownloadItem. CompleteDownload() will run the most recent
 // callback set.
-class DownloadCompletionBlocker : public content::DownloadItem::ExternalData {
+class DownloadCompletionBlocker : public base::SupportsUserData::Data {
  public:
   DownloadCompletionBlocker();
   virtual ~DownloadCompletionBlocker();
 
   bool is_complete() const { return is_complete_; }
 
   void set_callback(const base::Closure& callback) {
     if (!is_complete())
       callback_ = callback;
   }
 
   // Mark this download item as complete with respect to this blocker. (Other
   // blockers may continue to block the item.) Run |callback_|. This method may
   // only be called once, so |callback_| will only be called once.  Subclasses
   // must call the base implementation if they override this method.
   virtual void CompleteDownload();
 
  private:
   bool is_complete_;
   base::Closure callback_;
 
   DISALLOW_COPY_AND_ASSIGN(DownloadCompletionBlocker);
 };
 
 #endif  // CHROME_BROWSER_DOWNLOAD_DOWNLOAD_COMPLETION_BLOCKER_H_