Add the CustomElementsRegistry interface behind the flag

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

Index: third_party/WebKit/Source/core/dom/custom/CustomElementsRegistry.h
diff --git a/third_party/WebKit/Source/core/dom/custom/CustomElementsRegistry.h b/third_party/WebKit/Source/core/dom/custom/CustomElementsRegistry.h
new file mode 100644
index 0000000000000000000000000000000000000000..2d78fdf14ae2c61e98158f65c52d807b868eeef2
--- /dev/null
+++ b/third_party/WebKit/Source/core/dom/custom/CustomElementsRegistry.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 CustomElementsRegistry_h
+#define CustomElementsRegistry_h
+
+#include "bindings/core/v8/ScriptWrappable.h"
+#include "core/CoreExport.h"
+
+namespace blink {
+
+class ElementRegistrationOptions;
+class ExceptionState;
+class ScriptState;
+class ScriptValue;
+
+class CORE_EXPORT CustomElementsRegistry final
+    : public GarbageCollected<CustomElementsRegistry>
+    , public ScriptWrappable {
+    DEFINE_WRAPPERTYPEINFO();
+public:
+    static CustomElementsRegistry* create()
+    {
+        return new CustomElementsRegistry();
+    }
+
+    void define(ScriptState*, const AtomicString& name,
+        const ScriptValue& constructor, const ElementRegistrationOptions&,
+        ExceptionState&);
+
+    DECLARE_TRACE();
+
+private:
+    CustomElementsRegistry();
+};
+
+} // namespace blink
+
+#endif // CustomElementsRegistry_h