Remove ErrorEventInit's error's null default value

Per the spec change [1], this removes ErrorEventInit's default value
of null, which makes the error attribute have no null default.

This change has already shipped on WebKit and Gecko, and the change
makes Chromium match the spec and other implementations. This is tested
by this WPT:

wpt/html/webappapis/scripting/events/event-handler-processing-algorithm-error/document-synthetic-errorevent.html

[1] whatwg/html#7983

Fixed: 1332448
Change-Id: Ia64a62e8c5739ab278016a94b0c3b0419d6a68f6
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/3749784
Reviewed-by: Joey Arhar <jarhar@chromium.org>
Commit-Queue: Joey Arhar <jarhar@chromium.org>
Auto-Submit: Mason Freed <masonf@chromium.org>
Cr-Commit-Position: refs/heads/main@{#1021841}

html/webappapis/scripting/events/event-handler-processing-algorithm-error/document-synthetic-errorevent.html

@@ -41,4 +41,20 @@
   assert_equals(e.colno, 0);
   assert_equals(e.error, undefined);
 }, "Initial values of ErrorEvent members")
+
+test(() => {
+  const e = new ErrorEvent("error", {error : null});
+  assert_equals(e.error, null);
+}, "error member can be set to null")
+
+test(() => {
+  const e = new ErrorEvent("error", {error : undefined});
+  assert_equals(e.error, undefined);
+}, "error member can be set to undefined")
+
+test(() => {
+  const e = new ErrorEvent("error", {error : "foo"});
+  assert_equals(e.error, "foo");
+}, "error member can be set to arbitrary")
+
 </script>