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

Source/core/dom/custom/CustomElementMicrotaskDispatcher.h

 // Copyright 2014 The Chromium Authors. All rights reserved.
 // Use of this source code is governed by a BSD-style license that can be
 // found in the LICENSE file.
 
 #ifndef CustomElementMicrotaskDispatcher_h
 #define CustomElementMicrotaskDispatcher_h
 
 #include "core/dom/custom/CustomElementMicrotaskQueue.h"
 #include "wtf/Noncopyable.h"
 #include "wtf/PassOwnPtr.h"
@@ skipping 13 matching lines @@
 
     static CustomElementMicrotaskDispatcher& instance();
 
     void enqueue(HTMLImportLoader*, PassOwnPtr<CustomElementMicrotaskStep>);
     void enqueue(CustomElementCallbackQueue*);
 
     void importDidFinish(CustomElementMicrotaskImportStep*);
 
     bool elementQueueIsEmpty() { return m_elements.isEmpty(); }
 
+#if !defined(NDEBUG)
+    void show();
+#endif
+
 private:
     CustomElementMicrotaskDispatcher();
 
     void ensureMicrotaskScheduled();
 
     static void dispatch();
     void doDispatch();
 
     bool m_hasScheduledMicrotask;
     enum {
         Quiescent,
         Resolving,
         DispatchingCallbacks
     } m_phase;
 
     RefPtr<CustomElementMicrotaskQueue> m_resolutionAndImports;
     Vector<CustomElementCallbackQueue*> m_elements;
 };
 
 }
 
 #endif // CustomElementMicrotaskDispatcher_h