Make DownloadItem derive SupportsUserData instead of re-implementing it.

chrome/browser/extensions/webstore_installer.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_EXTENSIONS_WEBSTORE_INSTALLER_H_
 #define CHROME_BROWSER_EXTENSIONS_WEBSTORE_INSTALLER_H_
 
 #include <string>
 #include <vector>
 
@@ skipping 37 matching lines @@
                                            const std::string& error) = 0;
 
    protected:
     virtual ~Delegate() {}
   };
 
   // Contains information about what parts of the extension install process can
   // be skipped or modified. If one of these is present, it means that a CRX
   // download was initiated by WebstoreInstaller. The Approval instance should
   // be checked further for additional details.
-  struct Approval : public content::DownloadItem::ExternalData {
+  struct Approval : public base::SupportsUserData::Data {
     static scoped_ptr<Approval> CreateWithInstallPrompt(Profile* profile);
     static scoped_ptr<Approval> CreateWithNoInstallPrompt(
         Profile* profile,
         const std::string& extension_id,
         scoped_ptr<base::DictionaryValue> parsed_manifest);
 
     virtual ~Approval();
 
     // The extension id that was approved for installation.
     std::string extension_id;
@@ skipping 88 matching lines @@
   // transitions to state REMOVING.
   content::DownloadItem* download_item_;
   int flags_;
   scoped_ptr<Approval> approval_;
   GURL download_url_;
 };
 
 }  // namespace extensions
 
 #endif  // CHROME_BROWSER_EXTENSIONS_WEBSTORE_INSTALLER_H_