Test that custom element callback fires before DOMContentLoaded event.

LayoutTests/fast/dom/custom/callback-timing.html

+<script src='../../../resources/js-test.js'></script>
+<x-foo></x-foo>
+<script>
+description('Ensure that custom element callback fires before DOMContentLoaded event');
+
+var eventlog = [];
+
+addEventListener('DOMContentLoaded', function(e) {
+    eventlog.push('DOMContentLoaded event');
+});
+addEventListener('load', function(e) {
+    eventlog.push('load event');
+
+    shouldBe("eventlog[0]", "'created x-foo'");
+    shouldBe("eventlog[1]", "'DOMContentLoaded event'");
+    shouldBe("eventlog[2]", "'load event'");
+});
+var proto = Object.create(HTMLElement.prototype);
+proto.createdCallback = function() {
+    eventlog.push('created ' + this.localName);
+};
+document.registerElement('x-foo', {prototype: proto});
+</script>