Prevent recursion in critical part of CustomElementRegistry::define.

third_party/WebKit/Source/core/dom/custom/CustomElementRegistry.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 CustomElementRegistry_h
 #define CustomElementRegistry_h
 
 #include "base/gtest_prod_util.h"
 #include "bindings/core/v8/ScriptPromise.h"
 #include "bindings/core/v8/ScriptWrappable.h"
 #include "core/CoreExport.h"
 #include "platform/heap/Handle.h"
-#include "wtf/HashSet.h"
 #include "wtf/Noncopyable.h"
 #include "wtf/text/AtomicString.h"
 #include "wtf/text/AtomicStringHash.h"
 
 namespace blink {
 
 class CustomElementDefinition;
 class CustomElementDefinitionBuilder;
 class CustomElementDescriptor;
 class Element;
@@ skipping 52 matching lines @@
     friend class CustomElementRegistryTest;
 
     CustomElementRegistry(const LocalDOMWindow*);
 
     bool v0NameIsDefined(const AtomicString& name);
 
     void collectCandidates(
         const CustomElementDescriptor&,
         HeapVector<Member<Element>>*);
 
-    class NameIsBeingDefined;
+    class ElementDefinitionIsRunning;
+    bool m_elementDefinitionIsRunning;
 
-    HashSet<AtomicString> m_namesBeingDefined;
     using DefinitionMap =
         HeapHashMap<AtomicString, Member<CustomElementDefinition>>;
     DefinitionMap m_definitions;
 
     Member<const LocalDOMWindow> m_owner;
 
     using V0RegistrySet = HeapHashSet<WeakMember<V0CustomElementRegistrationContext>>;
     Member<V0RegistrySet> m_v0;
 
     using UpgradeCandidateSet = HeapHashSet<WeakMember<Element>>;
     using UpgradeCandidateMap = HeapHashMap<
         AtomicString,
         Member<UpgradeCandidateSet>>;
     Member<UpgradeCandidateMap> m_upgradeCandidates;
 
     using WhenDefinedPromiseMap =
         HeapHashMap<AtomicString, Member<ScriptPromiseResolver>>;
     WhenDefinedPromiseMap m_whenDefinedPromiseMap;
 };
 
 } // namespace blink
 
 #endif // CustomElementRegistry_h