Add testing of :defined prior to call to super()

Per the conversation here [1], there is a desire to add more
testing of the :defined pseudo state, prior to the call to super().
Note that `this` is not accessible prior to super(), so the
instance itself is used. Also note that :defined already does not
match anywhere inside the constructor, for upgrades.

[1] https://github.com/whatwg/dom/pull/894#discussion_r495092027

Bug: 1042130
Change-Id: I2372900981247ea5624e737d2597d004398de477
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2431558
Auto-Submit: Mason Freed <masonfreed@chromium.org>
Commit-Queue: Kouhei Ueno <kouhei@chromium.org>
Reviewed-by: Kouhei Ueno <kouhei@chromium.org>
Cr-Commit-Position: refs/heads/master@{#810991}

third_party/blink/web_tests/external/wpt/custom-elements/pseudo-class-defined.html

@@ -96,14 +96,17 @@ test(function () {
     var log = [];
     var instance = document.createElement('my-custom-element-2');
     document.body.appendChild(instance);
+    assert_false(instance.matches(":defined"), "Prior to definition, instance should not match :defined");
     customElements.define('my-custom-element-2',class extends HTMLElement {
         constructor() {
+            assert_false(instance.matches(":defined"), "During construction, prior to super(), instance should not match :defined");
             super();
             log.push([this, 'begin']);
-            assert_false(this.matches(":defined"), "During construction, this should not match :defined");
+            assert_false(this.matches(":defined"), "During construction, after super(), this should not match :defined");
             log.push([this, 'end']);
         }
     });
+    assert_true(instance.matches(":defined"), "After construction, instance should match :defined");
     assert_equals(log.length, 2);
     assert_array_equals(log[0], [instance, 'begin']);
     assert_array_equals(log[1], [instance, 'end']);