Add a const overload to DownloadItem::GetExternalData()

content/browser/download/download_item_impl.cc

Index: content/browser/download/download_item_impl.cc
diff --git a/content/browser/download/download_item_impl.cc b/content/browser/download/download_item_impl.cc
index dd08306e35e0f91cb76e444afd46fb453847da30..a8341b1daafb70b47fec34344ce203209beac0d3 100644
--- a/content/browser/download/download_item_impl.cc
+++ b/content/browser/download/download_item_impl.cc
@@ -1131,9 +1131,17 @@ 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(