Make customElements.define faster

third_party/WebKit/Source/platform/bindings/V8PerContextData.h

 /*
  * Copyright (C) 2012 Google Inc. All rights reserved.
  *
  * Redistribution and use in source and binary forms, with or without
  * modification, are permitted provided that the following conditions are
  * met:
  *
  *     * Redistributions of source code must retain the above copyright
  * notice, this list of conditions and the following disclaimer.
  *     * Redistributions in binary form must reproduce the above
@@ skipping 79 matching lines @@
 
   // Gets the constructor and prototype for a type, if they have already been
   // created. Returns true if they exist, and sets the existing values in
   // |prototypeObject| and |interfaceObject|. Otherwise, returns false, and the
   // values are set to empty objects (non-null).
   bool GetExistingConstructorAndPrototypeForType(
       const WrapperTypeInfo*,
       v8::Local<v8::Object>* prototype_object,
       v8::Local<v8::Function>* interface_object);
 
+  // Gets a Private to store custom element definition IDs on a
+  // function in use as a custom element constructor in this context.
+  v8::Local<v8::Private> GetPrivateCustomElementDefinitionId() {
+    if (UNLIKELY(private_custom_element_definition_id_.IsEmpty())) {
+      private_custom_element_definition_id_.Set(isolate_,
+                                                v8::Private::New(isolate_));
+    }
+    return private_custom_element_definition_id_.NewLocal(isolate_);
+  }
+
   void AddCustomElementBinding(std::unique_ptr<V0CustomElementBinding>);
 
   V8DOMActivityLogger* ActivityLogger() const { return activity_logger_; }
   void SetActivityLogger(V8DOMActivityLogger* activity_logger) {
     activity_logger_ = activity_logger;
   }
 
   // Garbage collected classes that use V8PerContextData to hold an instance
   // should subclass Data, and use addData / clearData / getData to manage the
   // instance.
@@ skipping 19 matching lines @@
 
   typedef V8GlobalValueMap<const WrapperTypeInfo*, v8::Function, v8::kNotWeak>
       ConstructorMap;
   ConstructorMap constructor_map_;
 
   std::unique_ptr<gin::ContextHolder> context_holder_;
 
   ScopedPersistent<v8::Context> context_;
   ScopedPersistent<v8::Value> error_prototype_;
 
+  // One function may be defined as a custom element constructor
+  // across multiple contexts simultaneously. Each context tags the
+  // constructor with a different Private to map the constructor back
+  // to its definition.

[haraken, 2017/04/28 08:49:30]

Can we use V8PrivateProperty instead?

+  ScopedPersistent<v8::Private> private_custom_element_definition_id_;
+
   typedef Vector<std::unique_ptr<V0CustomElementBinding>>
       V0CustomElementBindingList;
   V0CustomElementBindingList custom_element_bindings_;
 
   // This is owned by a static hash map in V8DOMActivityLogger.
   V8DOMActivityLogger* activity_logger_;
 
   using DataMap = PersistentHeapHashMap<const char*, Member<Data>>;
   DataMap data_map_;
 };
 
 }  // namespace blink
 
 #endif  // V8PerContextData_h