If 'is' is not null and custom element definition not found, NotFoundError should be thrown

third_party/WebKit/Source/core/dom/custom/V0CustomElementRegistrationContext.h

 /*
  * Copyright (C) 2013 Google Inc. All rights reserved.
  *
  * Redistribution and use in source and binary forms, with or without
  * modification, are permitted provided that the following conditions
  * are met:
  *
  * 1. Redistributions of source code must retain the above copyright
  *    notice, this list of conditions and the following disclaimer.
  * 2. Redistributions in binary form must reproduce the above copyright
@@ skipping 50 matching lines @@
 
   Element* createCustomTagElement(Document&, const QualifiedName&);
   static void setIsAttributeAndTypeExtension(Element*,
                                              const AtomicString& type);
   static void setTypeExtension(Element*, const AtomicString& type);
 
   void resolve(Element*, const V0CustomElementDescriptor&);
 
   bool nameIsDefined(const AtomicString& name) const;
   void setV1(const CustomElementRegistry*);
+  bool v1NameIsDefined(const AtomicString& name) const;
 
   DECLARE_TRACE();
 
  protected:
   V0CustomElementRegistrationContext();
 
   // Instance creation
   void didGiveTypeExtension(Element*, const AtomicString& type);
 
  private:
   void resolveOrScheduleResolution(Element*, const AtomicString& typeExtension);
 
   V0CustomElementRegistry m_registry;
 
   // Element creation
   Member<V0CustomElementUpgradeCandidateMap> m_candidates;
 };
 
 }  // namespace blink
 
 #endif  // V0CustomElementRegistrationContext_h