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

Source/core/html/imports/HTMLImportLoader.cpp

 /*
  * Copyright (C) 2013 Google Inc. All rights reserved.
  *
  * Redistribution and use in source and binary forms, with or without
  * modification, are permitted provided that the following conditions are
  * met:
  *
  *     * Redistributions of source code must retain the above copyright
  * notice, this list of conditions and the following disclaimer.
  *     * Redistributions in binary form must reproduce the above
@@ skipping 28 matching lines @@
 #include "core/html/imports/HTMLImportsController.h"
 #include "core/loader/DocumentWriter.h"
 #include "platform/network/ContentSecurityPolicyResponseHeaders.h"
 
 
 namespace WebCore {
 
 HTMLImportLoader::HTMLImportLoader(HTMLImportsController* controller)
     : m_controller(controller)
     , m_state(StateLoading)
-    , m_microtaskQueue(CustomElementMicrotaskQueue::create())
 {
 }
 
 HTMLImportLoader::~HTMLImportLoader()
 {
     clear();
 }
 
 void HTMLImportLoader::importDestroyed()
 {
@@ skipping 156 matching lines @@
     }
 
     return true;
 }
 
 PassRefPtr<CustomElementMicrotaskQueue> HTMLImportLoader::microtaskQueue() const
 {
     return m_microtaskQueue;
 }
 
+PassRefPtr<CustomElementMicrotaskQueue> HTMLImportLoader::ensureMicrotaskQueue()
+{
+    if (!m_microtaskQueue)
+        m_microtaskQueue = CustomElementMicrotaskQueue::create();
+    return m_microtaskQueue;
+}
+
 } // namespace WebCore