Add a scoped context object for synchronous CE creation

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

Index: third_party/WebKit/Source/core/frame/LocalDOMWindow.h
diff --git a/third_party/WebKit/Source/core/frame/LocalDOMWindow.h b/third_party/WebKit/Source/core/frame/LocalDOMWindow.h
index 648f98cd1698dae508f35600eeb59d8a310d15f8..b27641e952d7ebc872751687f047575a44197ce5 100644
--- a/third_party/WebKit/Source/core/frame/LocalDOMWindow.h
+++ b/third_party/WebKit/Source/core/frame/LocalDOMWindow.h
@@ -211,6 +211,9 @@ public:
 
     FloatSize getViewportSize(IncludeScrollbarsInRect) const;
 
+    void setCustomElementCreationContext(Document*);
+    Document* customElementCreationContext() const;
+
 protected:
     // EventTarget overrides.
     void addedEventListener(const AtomicString& eventType, RegisteredEventListener&) override;
@@ -267,6 +270,8 @@ private:
 
     HeapHashSet<Member<PostMessageTimer>> m_postMessageTimers;
     HeapHashSet<WeakMember<EventListenerObserver>> m_eventListenerObservers;
+
+    Member<Document> m_customElementCreationContext = nullptr;
 };
 
 DEFINE_TYPE_CASTS(LocalDOMWindow, DOMWindow, x, x->isLocalDOMWindow(), x.isLocalDOMWindow());