Custom Elements: add prototype check for "failed" elements

third_party/WebKit/LayoutTests/custom-elements/spec/state-failed-upgrade.html

Index: third_party/WebKit/LayoutTests/custom-elements/spec/state-failed-upgrade.html
diff --git a/third_party/WebKit/LayoutTests/custom-elements/spec/state-failed-upgrade.html b/third_party/WebKit/LayoutTests/custom-elements/spec/state-failed-upgrade.html
index 995ee272ea2a87d5ddfcf3a51a08282c01718bd2..fff117f7b99ae8ba6e90bcedcaa0faa57e1e16cf 100644
--- a/third_party/WebKit/LayoutTests/custom-elements/spec/state-failed-upgrade.html
+++ b/third_party/WebKit/LayoutTests/custom-elements/spec/state-failed-upgrade.html
@@ -21,6 +21,8 @@ test_with_window(w => {
       throw new Error();
     }
     connectedCallback() {
+      // TODO(davaajav): remove the failure expectation when this issue is closed
+      // https://github.com/w3c/webcomponents/issues/563
       assert_unreached('connectedCallback should not be invoked if constructor threw');
     }
   });
@@ -36,6 +38,15 @@ test_with_window(w => {
   // "failed" is not "defined"
   // https://dom.spec.whatwg.org/#concept-element-defined
   assert_false(element.matches(':defined'));
+
+  // "failed" element should implement HTMLUnknownElement only in "creating an element for a token"
+  // https://html.spec.whatwg.org/#create-an-element-for-the-token
+  assert_equals(Object.getPrototypeOf(element), w.HTMLElement.prototype);
+
+  // 2. If element's custom element state is "failed", then abort these steps.
+  // https://html.spec.whatwg.org/multipage/scripting.html#upgrades
+  container.appendChild(element);
+  assert_equals(constructorCount, 1, 'constructor should be invoked once');
 });
 </script>
 </body>