Custom Elements: layout tests for custom built-in elements definition

third_party/WebKit/LayoutTests/custom-elements/spec/define-builtin-element-expected.txt

+This is a testharness.js-based test.
+FAIL Element interface for extends is not valid custom element name assert_throws: having valid custon element name element interface (a-a) for extends should throw a NotSupportedError function "() => {
+    try {
+      func.call(this);
+    } catch(e) {
+      exception = e;
+      throw e;
+    }
+  }" did not throw
+FAIL Element interface for extends defined in specification assert_throws: having element interface for extends (bgsound) undefined in specs should throw a NotSupportedError function "() => {
+    try {
+      func.call(this);
+    } catch(e) {
+      exception = e;
+      throw e;
+    }
+  }" did not throw
+FAIL localName set to element interface for extends promise_test: Unhandled rejection with value: object "TypeError: Illegal constructor"
+Harness: the test ran to completion.
+