Add a scoped context object for synchronous CE creation

third_party/WebKit/Source/bindings/core/v8/custom/V8HTMLElementCustom.cpp

Index: third_party/WebKit/Source/bindings/core/v8/custom/V8HTMLElementCustom.cpp
diff --git a/third_party/WebKit/Source/bindings/core/v8/custom/V8HTMLElementCustom.cpp b/third_party/WebKit/Source/bindings/core/v8/custom/V8HTMLElementCustom.cpp
index 77e327f20115c9ae3ce0ec6993fa13938d6998ae..57ba51ac259f16fb215d0c4ddd3679915249c36f 100644
--- a/third_party/WebKit/Source/bindings/core/v8/custom/V8HTMLElementCustom.cpp
+++ b/third_party/WebKit/Source/bindings/core/v8/custom/V8HTMLElementCustom.cpp
@@ -59,9 +59,13 @@ void V8HTMLElement::constructorCustom(
         // TODO(kojii): When HTMLElementFactory has an option not to queue
         // upgrade, call that instead of HTMLElement. HTMLElement is enough
         // for now, but type extension will require HTMLElementFactory.
+        Document* context = window->customElementCreationContext();
+        if (!context)
+            context = window->document();
+
         element = HTMLElement::create(
             QualifiedName(nullAtom, definition->descriptor().localName(), HTMLNames::xhtmlNamespaceURI),
-            *window->document());
+            *context);
         // TODO(davaajav): write this as one call to setCustomElementState instead of two
         element->setCustomElementState(CustomElementState::Undefined);
         element->setCustomElementDefinition(definition);