Let unresolved custom element go through CustomElementCallbackQueue.

Source/core/dom/custom/CustomElementCallbackInvocation.h

Index: Source/core/dom/custom/CustomElementCallbackInvocation.h
diff --git a/Source/core/dom/custom/CustomElementCallbackInvocation.h b/Source/core/dom/custom/CustomElementCallbackInvocation.h
index 097e0918b08a0259c8efeef1c638ceccfac429b4..80a0b347430c097540c7e1466a426bd6ac72a216 100644
--- a/Source/core/dom/custom/CustomElementCallbackInvocation.h
+++ b/Source/core/dom/custom/CustomElementCallbackInvocation.h
@@ -32,6 +32,7 @@
 #define CustomElementCallbackInvocation_h
 
 #include "core/dom/custom/CustomElementLifecycleCallbacks.h"
+#include "core/dom/custom/CustomElementProcessingStep.h"
 #include "wtf/PassOwnPtr.h"
 #include "wtf/PassRefPtr.h"
 #include "wtf/RefPtr.h"
@@ -39,18 +40,13 @@
 
 namespace WebCore {
 
-class Element;
-
-class CustomElementCallbackInvocation {
+// FIXME: Rename to CustomElementCallbackStep
+class CustomElementCallbackInvocation : public CustomElementProcessingStep {
     WTF_MAKE_NONCOPYABLE(CustomElementCallbackInvocation);
 public:
     static PassOwnPtr<CustomElementCallbackInvocation> createInvocation(PassRefPtr<CustomElementLifecycleCallbacks>, CustomElementLifecycleCallbacks::CallbackType);
     static PassOwnPtr<CustomElementCallbackInvocation> createAttributeChangedInvocation(PassRefPtr<CustomElementLifecycleCallbacks>, const AtomicString& name, const AtomicString& oldValue, const AtomicString& newValue);
 
-    virtual ~CustomElementCallbackInvocation() { }
-    virtual void dispatch(Element*) = 0;
-    virtual bool isCreated() const { return false; }
-
 protected:
     CustomElementCallbackInvocation(PassRefPtr<CustomElementLifecycleCallbacks> callbacks)
         : m_callbacks(callbacks)