Load offline page if reading list entry takes more than 1s to load.

ios/web/public/navigation_item.h

Index: ios/web/public/navigation_item.h
diff --git a/ios/web/public/navigation_item.h b/ios/web/public/navigation_item.h
index 48a62f7f1670d9851490510c0d8ed11d37207e52..4bfd430e77727bdc0c64ef3d93929e3a4fbdafe2 100644
--- a/ios/web/public/navigation_item.h
+++ b/ios/web/public/navigation_item.h
@@ -117,6 +117,8 @@ class NavigationItem : public base::SupportsUserData {
   // Adds headers from |additional_headers| to the item's http request headers.
   // Existing headers with the same key will be overridden.
   virtual void AddHttpRequestHeaders(NSDictionary* additional_headers) = 0;
+
+  virtual NSDictionary* GetExtraData() const = 0;
 };
 
 }  // namespace web