REGRESSION(r171966): Custom elements in async imports don't get upgrade.

LayoutTests/http/tests/htmlimports/resources/import-custom-element-helper.js

+
+function registerTestingCustomElement(tagName) {
+    var definition = function() {};
+    definition.prototype = Object.create(HTMLElement.prototype);
+    definition.prototype.createdCallback = function() {
+        if (typeof this.constructor.ids === "undefined")
+            this.constructor.ids = [];
+        this.constructor.ids.push(this.id);
+     }
+
+    var ctor = document.registerElement(tagName, definition);
+    return ctor;
+}
+
+function ImportTestLatch(test, count) {
+    this.loaded = function() {
+        count--;
+        if (!count)
+            test();
+    };
+}