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

Source/core/html/imports/HTMLImport.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 36 matching lines @@
 bool HTMLImport::precedes(HTMLImport* import)
 {
     for (HTMLImport* i = this; i; i = traverseNext(i)) {
         if (i == import)
             return true;
     }
 
     return false;
 }
 
+bool HTMLImport::formsCycle() const
+{
+    for (const HTMLImport* i = this->parent(); i; i = i->parent()) {
+        if (i->document() == this->document())
+            return true;
+    }
+
+    return false;
+
+}
+
 void HTMLImport::appendImport(HTMLImport* child)
 {
     appendChild(child);
 
     // This prevents HTML parser from going beyond the
     // blockage line before the precise state is computed by recalcState().
     if (child->isSync())
         m_state = HTMLImportState::blockedState();
 
     stateWillChange();
@@ skipping 56 matching lines @@
         child->showTree(highlight, depth + 1);
 }
 
 void HTMLImport::showThis()
 {
     fprintf(stderr, "%p state=%d", this, m_state.peekValueForDebug());
 }
 #endif
 
 } // namespace WebCore