Add a scoped context object for synchronous CE creation

third_party/WebKit/Source/core/frame/LocalDOMWindow.cpp

Index: third_party/WebKit/Source/core/frame/LocalDOMWindow.cpp
diff --git a/third_party/WebKit/Source/core/frame/LocalDOMWindow.cpp b/third_party/WebKit/Source/core/frame/LocalDOMWindow.cpp
index 39cd651a3ef4a1919d2c72c2028031269cbfa32c..78e6ca802ed30069fc8ba9d2fd956f16dde774ff 100644
--- a/third_party/WebKit/Source/core/frame/LocalDOMWindow.cpp
+++ b/third_party/WebKit/Source/core/frame/LocalDOMWindow.cpp
@@ -332,6 +332,16 @@ void LocalDOMWindow::acceptLanguagesChanged()
     dispatchEvent(Event::create(EventTypeNames::languagechange));
 }
 
+void LocalDOMWindow::setCustomElementCreationContext(Document* context)
+{
+    m_customElementCreationContext = context;
+}
+
+Document* LocalDOMWindow::customElementCreationContext() const
+{
+    return m_customElementCreationContext;
+}
+
 Document* LocalDOMWindow::createDocument(const String& mimeType, const DocumentInit& init, bool forceXHTML)
 {
     Document* document = nullptr;
@@ -1534,6 +1544,7 @@ DEFINE_TRACE(LocalDOMWindow)
     visitor->trace(m_postMessageTimers);
     visitor->trace(m_visualViewport);
     visitor->trace(m_eventListenerObservers);
+    visitor->trace(m_customElementCreationContext);
     DOMWindow::trace(visitor);
     Supplementable<LocalDOMWindow>::trace(visitor);
 }