Add a const overload to DownloadItem::GetExternalData()

content/browser/download/download_item_impl.cc

 // 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.
 
 #include "content/browser/download/download_item_impl.h"
 
 #include <vector>
 
 #include "base/basictypes.h"
 #include "base/bind.h"
@@ skipping 1113 matching lines @@
   return last_reason_;
 }
 DownloadStateInfo DownloadItemImpl::GetStateInfo() const { return state_info_; }
 bool DownloadItemImpl::NeedsRename() const {
   return state_info_.target_name != full_path_.BaseName();
 }
 void DownloadItemImpl::MockDownloadOpenForTesting() { open_enabled_ = false; }
 
 DownloadItem::ExternalData*
 DownloadItemImpl::GetExternalData(const void* key) {
-  if (!ContainsKey(external_data_map_, key))
-    return NULL;
-  return external_data_map_[key];
+  // The behavior of the const overload is identical with the exception of the
+  // constness of |this| and the return value.
+  return const_cast<DownloadItem::ExternalData*>(
+      static_cast<const DownloadItemImpl&>(*this).GetExternalData(key));
+}
+
+const DownloadItem::ExternalData*
+DownloadItemImpl::GetExternalData(const void* key) const {
+  std::map<const void*, ExternalData*>::const_iterator it =
+      external_data_map_.find(key);
+  return (it == external_data_map_.end()) ? NULL : it->second;
 }
 
 void DownloadItemImpl::SetExternalData(
     const void* key, DownloadItem::ExternalData* data) {
   std::map<const void*, ExternalData*>::iterator it =
       external_data_map_.find(key);
 
   if (it == external_data_map_.end()) {
     external_data_map_[key] = data;
   } else if (it->second != data) {
     delete it->second;
     it->second = data;
   }
 }