Support Custom Elements v1 in attachShadow

third_party/WebKit/LayoutTests/shadow-dom/attach-shadow-safelist.html

Index: third_party/WebKit/LayoutTests/shadow-dom/attach-shadow-safelist.html
diff --git a/third_party/WebKit/LayoutTests/shadow-dom/attach-shadow-safelist.html b/third_party/WebKit/LayoutTests/shadow-dom/attach-shadow-safelist.html
index 3f2a9fc35b5259c032f03705364c0435f4241a1f..c9992b038161a3c74b5451a189a1e24be19699dc 100644
--- a/third_party/WebKit/LayoutTests/shadow-dom/attach-shadow-safelist.html
+++ b/third_party/WebKit/LayoutTests/shadow-dom/attach-shadow-safelist.html
@@ -41,4 +41,14 @@ test(() => {
     });
   });
 }, 'attachShadow should throw an exception for an element which is not in the safelist');
+
+test(() => {
+  class XFoo extends HTMLElement {
+    constructor() {
+      super();
+    }
+  }
+  window.customElements.define('x-foo', XFoo);
+  assert_true(new XFoo().attachShadow({mode: 'open'}) instanceof ShadowRoot);
+}, 'attachShadow should not fail for Custom Elements v1.');
 </script>