Let unresolved custom element go through CustomElementCallbackQueue.

Source/core/dom/custom/CustomElementProcessingStep.h

Index: Source/core/dom/custom/CustomElementProcessingStep.h
diff --git a/Source/core/dom/Microtask.h b/Source/core/dom/custom/CustomElementProcessingStep.h
similarity index 78%
copy from Source/core/dom/Microtask.h
copy to Source/core/dom/custom/CustomElementProcessingStep.h
index 27f72cbec01e5795593d3e545a3b5ca3e5c046fa..c92451bae82373f7dc25694b071d9fdbf1af27b5 100644
--- a/Source/core/dom/Microtask.h
+++ b/Source/core/dom/custom/CustomElementProcessingStep.h
@@ -28,19 +28,25 @@
  * 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