Clear the custom element's reaction queue if upgrade fails.

third_party/WebKit/LayoutTests/custom-elements/spec/resources/custom-elements-helpers.js

 function create_window_in_test(t, srcdoc) {
   let p = new Promise((resolve) => {
     let f = document.createElement('iframe');
     f.srcdoc = srcdoc ? srcdoc : '';
     f.onload = (event) => {
       let w = f.contentWindow;
       t.add_cleanup(() => f.parentNode && f.remove());
       resolve(w);
     };
     document.body.appendChild(f);
@@ skipping 47 matching lines @@
     errors.push(error);
     return true; // the error is handled
   };
   try {
     func();
   } catch (e) {
     assert_unreached(`should report, not throw, an exception: ${e}`);
   } finally {
     w.onerror = old_onerror;
   }
-  assert_equals(1, errors.length);
+  assert_equals(errors.length, 1, 'only one error should have been reported');
   // assert_throws has an error expectation matcher that can only be
   // accessed by throwing the error
   assert_throws(expected_error, () => { throw errors[0]; });
 }