Add a scoped context object for synchronous CE creation

third_party/WebKit/Source/core/dom/custom/CustomElementCreationScope.h

Index: third_party/WebKit/Source/core/dom/custom/CustomElementCreationScope.h
diff --git a/third_party/WebKit/Source/core/dom/custom/CustomElementCreationScope.h b/third_party/WebKit/Source/core/dom/custom/CustomElementCreationScope.h
new file mode 100644
index 0000000000000000000000000000000000000000..d8f2dbd120ef60530bc7804f55c7054b7d073412
--- /dev/null
+++ b/third_party/WebKit/Source/core/dom/custom/CustomElementCreationScope.h
@@ -0,0 +1,40 @@
+// Copyright 2016 The Chromium Authors. All rights reserved.
+// Use of this source code is governed by a BSD-style license that can be
+// found in the LICENSE file.
+
+#ifndef CustomElementCreationScope_h
+#define CustomElementCreationScope_h
+
+#include "core/frame/LocalDOMWindow.h"
+#include "platform/heap/Handle.h"
+
+namespace blink {
+
+class Document;
+
+class CustomElementCreationScope {
+    STACK_ALLOCATED();
+public:
+    explicit CustomElementCreationScope(Document* document)
+        : m_document(document)
+    {
+        if (LocalDOMWindow* window = m_document->executingWindow()) {
+            m_prev = window->customElementCreationContext();
+            window->setCustomElementCreationContext(m_document);
+        }
+    }
+
+    ~CustomElementCreationScope()
+    {
+        if (LocalDOMWindow* window = m_document->executingWindow())
+            window->setCustomElementCreationContext(m_prev);
+    }
+
+private:
+    Member<Document> m_document;
+    Member<Document> m_prev = nullptr;
+};
+
+} // namespace blink
+
+#endif // CustomElementCreationScope_h