Make customElements.define faster

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

Index: third_party/WebKit/Source/platform/bindings/V8PerContextData.h
diff --git a/third_party/WebKit/Source/platform/bindings/V8PerContextData.h b/third_party/WebKit/Source/platform/bindings/V8PerContextData.h
index b3bf2a737dc63c24d25e4d02826294997edc23d0..ae3229bb5e9448b02ffbe43524131f1c654d0fe6 100644
--- a/third_party/WebKit/Source/platform/bindings/V8PerContextData.h
+++ b/third_party/WebKit/Source/platform/bindings/V8PerContextData.h
@@ -97,6 +97,16 @@ class PLATFORM_EXPORT V8PerContextData final {
       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_; }
@@ -136,6 +146,12 @@ class PLATFORM_EXPORT V8PerContextData final {
   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_;