Prevent recursion in critical part of CustomElementRegistry::define.

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

Index: third_party/WebKit/Source/core/dom/custom/CustomElementRegistry.h
diff --git a/third_party/WebKit/Source/core/dom/custom/CustomElementRegistry.h b/third_party/WebKit/Source/core/dom/custom/CustomElementRegistry.h
index 338600944ff3b2f60806c750a00f21392d367ed8..04d145614f87ad644ff1abf47909d09c88c700b1 100644
--- a/third_party/WebKit/Source/core/dom/custom/CustomElementRegistry.h
+++ b/third_party/WebKit/Source/core/dom/custom/CustomElementRegistry.h
@@ -10,7 +10,6 @@
 #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"
@@ -83,9 +82,9 @@ private:
         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;