Store |detail| as a hidden value of custom event wrappers

third_party/WebKit/LayoutTests/fast/js/constructor-length.html

Index: third_party/WebKit/LayoutTests/fast/js/constructor-length.html
diff --git a/third_party/WebKit/LayoutTests/fast/js/constructor-length.html b/third_party/WebKit/LayoutTests/fast/js/constructor-length.html
index 56f1c991b291dde265427404cc106ab8a3a85300..f82f4ca2af0528cc5aaedb061b27266783936d04 100644
--- a/third_party/WebKit/LayoutTests/fast/js/constructor-length.html
+++ b/third_party/WebKit/LayoutTests/fast/js/constructor-length.html
@@ -12,6 +12,8 @@ shouldBe('AudioContext.length', '0');
 shouldBe('AutocompleteErrorEvent.length', '1');
 shouldBe('Blob.length', '0');
 shouldBe('CloseEvent.length', '1');
+// TODO(bashi): CustomEvent.length == 0 since it uses custom constructor.
+// Fix this regression once http://crbug.com/529941 is fixed.
 shouldBe('CustomEvent.length', '1');
 shouldBe('DOMFormData.length', '0');
 shouldBe('DOMParser.length', '0');