Let unresolved custom element go through CustomElementCallbackQueue.

Source/core/dom/custom/CustomElementProcessingStep.h

 /*
  * 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:
  *
  * 1. Redistributions of source code must retain the above copyright
  *    notice, this list of conditions and the following disclaimer.
  * 2. Redistributions in binary form must reproduce the above copyright
@@ skipping 10 matching lines @@
  * A PARTICULAR PURPOSE ARE DISCLAIMED. IN NO EVENT SHALL THE COPYRIGHT
  * OWNER OR CONTRIBUTORS BE LIABLE FOR ANY DIRECT, INDIRECT, INCIDENTAL,
  * SPECIAL, EXEMPLARY, OR CONSEQUENTIAL DAMAGES (INCLUDING, BUT NOT
  * LIMITED TO, PROCUREMENT OF SUBSTITUTE GOODS OR SERVICES; LOSS OF USE,
  * DATA, OR PROFITS; OR BUSINESS INTERRUPTION) HOWEVER CAUSED AND ON ANY
  * THEORY OF LIABILITY, WHETHER IN CONTRACT, STRICT LIABILITY, OR TORT
  * (INCLUDING NEGLIGENCE OR OTHERWISE) ARISING IN ANY WAY OUT OF THE USE
  * OF THIS SOFTWARE, EVEN IF ADVISED OF THE POSSIBILITY OF SUCH DAMAGE.
  */
 
-#ifndef Microtask_h
-#define Microtask_h
+#ifndef CustomElementProcessingStep_h
+#define CustomElementProcessingStep_h
+
+#include "wtf/Noncopyable.h"
 
 namespace WebCore {
 
-class Microtask {
+class Element;
+
+class CustomElementProcessingStep {
+    WTF_MAKE_NONCOPYABLE(CustomElementProcessingStep);
 public:
-    static void performCheckpoint();
+    CustomElementProcessingStep() { }
 
-private:
-    explicit Microtask();
+    virtual ~CustomElementProcessingStep() { }
+    virtual void dispatch(Element*) = 0;
+    virtual bool isCreated() const { return false; }
 };
 
 }
 
-#endif // Microtask_h
+#endif // CustomElementProcessingStep_h