Clean up threading model of external extension providers

chrome/browser/extensions/external_registry_extension_loader_win.h

+// Copyright (c) 2010 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 CHROME_BROWSER_EXTENSIONS_EXTERNAL_REGISTRY_EXTENSION_LOADER_WIN_H_
+#define CHROME_BROWSER_EXTENSIONS_EXTERNAL_REGISTRY_EXTENSION_LOADER_WIN_H_
+#pragma once
+
+#include "chrome/browser/extensions/external_extension_loader.h"
+
+class ExternalRegistryExtensionLoader : public ExternalExtensionLoader {
+ public:
+  ExternalRegistryExtensionLoader() {}
+
+ protected:
+  virtual void StartLoading();
+
+ private:
+  friend class base::RefCountedThreadSafe<ExternalExtensionLoader>;
+
+  virtual ~ExternalRegistryExtensionLoader() {}
+
+  void LoadOnFileThread();
+
+  DISALLOW_COPY_AND_ASSIGN(ExternalRegistryExtensionLoader);
+};
+
+#endif  // CHROME_BROWSER_EXTENSIONS_EXTERNAL_REGISTRY_EXTENSION_LOADER_WIN_H_