Add a const overload to DownloadItem::GetExternalData()

content/public/browser/download_item.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.
 //
 // Each download is represented by a DownloadItem, and all DownloadItems
 // are owned by the DownloadManager which maintains a global list of all
 // downloads. DownloadItems are created when a user initiates a download,
 // and exist for the duration of the browser life time.
 //
 // Download observers:
@@ skipping 326 matching lines @@
   // static char subsystem_specific_string[] = ".."; defined
   // in the subsystem, but the only requirement of this interface
   // is that the key be unique over all data stored with this
   // DownloadItem.
   //
   // Note that SetExternalData takes ownership of the
   // passed object; it will be destroyed when the DownloadItem is.
   // If an object is already held by the DownloadItem associated with
   // the passed key, it will be destroyed if overwriten by a new pointer
   // (overwrites by the same pointer are ignored).
-  virtual ExternalData* GetExternalData(const void* key) = 0;
+  virtual       ExternalData* GetExternalData(const void* key) = 0;
+  virtual const ExternalData* GetExternalData(const void* key) const = 0;
   virtual void SetExternalData(const void* key, ExternalData* data) = 0;
 
   virtual std::string DebugString(bool verbose) const = 0;
 
   virtual void MockDownloadOpenForTesting() = 0;
 };
 
 }  // namespace content
 
 #endif  // CONTENT_PUBLIC_BROWSER_DOWNLOAD_ITEM_H_