servoshell: Port desktop servoshell to use delegate API (#35284)

Signed-off-by: Martin Robinson <mrobinson@igalia.com>
Co-authored-by: Delan Azabani <dazabani@igalia.com>
Co-authored-by: Mukilan Thiyagarajan <mukilan@igalia.com>

ports/servoshell/desktop/dialog.rs

@@ -92,4 +92,8 @@ impl Dialog {
             },
         }
     }
+
+    pub(crate) fn is_file_dialog(&self) -> bool {
+        matches!(self, Dialog::File(..))
+    }
 }