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

third_party/WebKit/Source/core/dom/custom/CustomElementReactionStack.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 CustomElementReactionStack_h
 #define CustomElementReactionStack_h
 
 #include "core/CoreExport.h"
 #include "platform/heap/Handle.h"
 #include "wtf/Noncopyable.h"
@@ skipping 11 matching lines @@
 
  public:
   CustomElementReactionStack();
 
   DECLARE_TRACE();
 
   void push();
   void popInvokingReactions();
   void enqueueToCurrentQueue(Element*, CustomElementReaction*);
   void enqueueToBackupQueue(Element*, CustomElementReaction*);
+  void clearQueue(Element*);
 
   static CustomElementReactionStack& current();
 
  private:
+  friend class CustomElementReactionStackTestSupport;
+
   using ElementReactionQueueMap =
       HeapHashMap<Member<Element>, Member<CustomElementReactionQueue>>;
   ElementReactionQueueMap m_map;
 
   using ElementQueue = HeapVector<Member<Element>, 1>;
   HeapVector<Member<ElementQueue>> m_stack;
   Member<ElementQueue> m_backupQueue;
 
   void invokeBackupQueue();
   void invokeReactions(ElementQueue&);
   void enqueue(Member<ElementQueue>&, Element*, CustomElementReaction*);
 };
 
+class CORE_EXPORT CustomElementReactionStackTestSupport final {
+ private:
+  friend class ResetCustomElementReactionStackForTest;
+
+  CustomElementReactionStackTestSupport() = delete;
+  static CustomElementReactionStack* setCurrentForTest(
+      CustomElementReactionStack*);
+};
+
 }  // namespace blink
 
 #endif  // CustomElementReactionStack_h