Add a const overload to DownloadItem::GetExternalData()

content/browser/download/download_item_impl.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 CONTENT_BROWSER_DOWNLOAD_DOWNLOAD_ITEM_IMPL_H_
 #define CONTENT_BROWSER_DOWNLOAD_DOWNLOAD_ITEM_IMPL_H_
 #pragma once
 
 #include <string>
 
@@ skipping 191 matching lines @@
   virtual content::BrowserContext* GetBrowserContext() const OVERRIDE;
   virtual content::WebContents* GetWebContents() const OVERRIDE;
   virtual FilePath GetTargetFilePath() const OVERRIDE;
   virtual FilePath GetFileNameToReportUser() const OVERRIDE;
   virtual FilePath GetUserVerifiedFilePath() const OVERRIDE;
   virtual bool NeedsRename() const OVERRIDE;
   virtual void OffThreadCancel(DownloadFileManager* file_manager) OVERRIDE;
   virtual std::string DebugString(bool verbose) const OVERRIDE;
   virtual void MockDownloadOpenForTesting() OVERRIDE;
   virtual ExternalData* GetExternalData(const void* key) OVERRIDE;
+  virtual const ExternalData* GetExternalData(const void* key) const OVERRIDE;
   virtual void SetExternalData(const void* key, ExternalData* data) OVERRIDE;
 
  private:
   // Construction common to all constructors. |active| should be true for new
   // downloads and false for downloads from the history.
   // |download_type| indicates to the net log system what kind of download
   // this is.
   void Init(bool active, download_net_logs::DownloadType download_type);
 
   // Internal helper for maintaining consistent received and total sizes, and
@@ skipping 159 matching lines @@
   // are owned by the DownloadItemImpl.
   std::map<const void*, ExternalData*> external_data_map_;
 
   // Net log to use for this download.
   const net::BoundNetLog bound_net_log_;
 
   DISALLOW_COPY_AND_ASSIGN(DownloadItemImpl);
 };
 
 #endif  // CONTENT_BROWSER_DOWNLOAD_DOWNLOAD_ITEM_IMPL_H_