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

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

Index: third_party/WebKit/Source/core/dom/custom/CustomElementReactionStack.h
diff --git a/third_party/WebKit/Source/core/dom/custom/CustomElementReactionStack.h b/third_party/WebKit/Source/core/dom/custom/CustomElementReactionStack.h
index d053446f994b925321a770254131bd82d950aae3..8245eef949fa69965dedd0fb142e1e42eb5b1b48 100644
--- a/third_party/WebKit/Source/core/dom/custom/CustomElementReactionStack.h
+++ b/third_party/WebKit/Source/core/dom/custom/CustomElementReactionStack.h
@@ -29,10 +29,13 @@ class CORE_EXPORT CustomElementReactionStack final
   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;
@@ -46,6 +49,15 @@ class CORE_EXPORT CustomElementReactionStack final
   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