Implement the script parts of custom element upgrade steps.

third_party/WebKit/Source/core/dom/custom/CustomElementDefinition.h

 // Copyright 2016 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 CustomElementDefinition_h
 #define CustomElementDefinition_h
 
 #include "core/CoreExport.h"
 #include "core/dom/custom/CustomElementDescriptor.h"
 #include "platform/heap/Handle.h"
@@ skipping 16 matching lines @@
 
     using ConstructionStack = HeapVector<Member<Element>, 1>;
     ConstructionStack& constructionStack()
     {
         return m_constructionStack;
     }
 
     void upgrade(Element*);
 
 protected:
-    // TODO(dominicc): Make this pure virtual when the script side is
-    // implemented.
-    virtual bool runConstructor(Element*);
+    virtual bool runConstructor(Element*) = 0;
 
 private:
     const CustomElementDescriptor m_descriptor;
     ConstructionStack m_constructionStack;
 };
 
 } // namespace blink
 
 #endif // CustomElementDefinition_h