Clear the custom element's reaction queue if upgrade fails.

third_party/WebKit/Source/core/dom/custom/CustomElementReactionQueue.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 CustomElementReactionQueue_h
 #define CustomElementReactionQueue_h
 
 #include "core/CoreExport.h"
 #include "platform/heap/Handle.h"
 #include "wtf/Noncopyable.h"
 
 namespace blink {
 
 class CustomElementReaction;
 class Element;
 
 class CORE_EXPORT CustomElementReactionQueue final
     : public GarbageCollectedFinalized<CustomElementReactionQueue> {
   WTF_MAKE_NONCOPYABLE(CustomElementReactionQueue);
 
  public:
   CustomElementReactionQueue();
   ~CustomElementReactionQueue();
 
   DECLARE_TRACE();
 
   void add(CustomElementReaction*);
   void invokeReactions(Element*);
   bool isEmpty() { return m_reactions.isEmpty(); }
+  void clear();
 
  private:
   HeapVector<Member<CustomElementReaction>, 1> m_reactions;
   size_t m_index;
 };
 
 }  // namespace blink
 
 #endif  // CustomElementReactionQueue_h