Add a const overload to DownloadItem::GetExternalData()

content/public/browser/download_item.h

Index: content/public/browser/download_item.h
diff --git a/content/public/browser/download_item.h b/content/public/browser/download_item.h
index 803c6bfc75d19646cbf75b1d1d36034a9fc6bafe..106cfb6ce974edb22809f449a223597321cd05e3 100644
--- a/content/public/browser/download_item.h
+++ b/content/public/browser/download_item.h
@@ -344,7 +344,8 @@ class CONTENT_EXPORT DownloadItem {
   // 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;