Let unresolved custom element go through CustomElementCallbackQueue.

Source/core/dom/custom/CustomElementCallbackScheduler.h

Index: Source/core/dom/custom/CustomElementCallbackScheduler.h
diff --git a/Source/core/dom/custom/CustomElementCallbackScheduler.h b/Source/core/dom/custom/CustomElementCallbackScheduler.h
index 0311d05662332ee6d59398ba6557952b2ebc8cc2..ed4aa18d6e12cc5cce1aa2b8d6a11b7131f2d158 100644
--- a/Source/core/dom/custom/CustomElementCallbackScheduler.h
+++ b/Source/core/dom/custom/CustomElementCallbackScheduler.h
@@ -39,6 +39,7 @@
 
 namespace WebCore {
 
+class CustomElementDescriptor;
 class CustomElementLifecycleCallbacks;
 class Element;
 
@@ -48,6 +49,7 @@ public:
     static void scheduleCreatedCallback(PassRefPtr<CustomElementLifecycleCallbacks>, PassRefPtr<Element>);
     static void scheduleEnteredViewCallback(PassRefPtr<CustomElementLifecycleCallbacks>, PassRefPtr<Element>);
     static void scheduleLeftViewCallback(PassRefPtr<CustomElementLifecycleCallbacks>, PassRefPtr<Element>);
+    static void scheduleResolutionStep(const CustomElementDescriptor&, PassRefPtr<Element>);
 
 protected:
     friend class CustomElementCallbackDispatcher;