fix: abort more descriptively for beforeunload (#49011)

Co-authored-by: trop[bot] <37223003+trop[bot]@users.noreply.github.com>
Co-authored-by: Shelley Vohr <shelley.vohr@gmail.com>

shell/browser/api/electron_api_web_contents.h

@@ -844,6 +844,8 @@ class WebContents final : public ExclusiveAccessContext,
   // that field to ensure the dtor destroys them in the right order.
   raw_ptr<WebContentsZoomController> zoom_controller_ = nullptr;
 
+  std::optional<GURL> pending_unload_url_ = std::nullopt;
+
   // Maps url to file path, used by the file requests sent from devtools.
   typedef std::map<std::string, base::FilePath> PathsMap;
   PathsMap saved_files_;