Clean up threading model of external extension providers

chrome/browser/extensions/external_pref_extension_provider.h

-// Copyright (c) 2009 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_PREF_EXTENSION_PROVIDER_H_
-#define CHROME_BROWSER_EXTENSIONS_EXTERNAL_PREF_EXTENSION_PROVIDER_H_
-#pragma once
-
-#include "chrome/browser/extensions/stateful_external_extension_provider.h"
-
-// A specialization of the ExternalExtensionProvider that uses a json file to
-// look up which external extensions are registered.
-// Instances of this class are expected to be created and destroyed on the UI
-// thread and they are expecting public method calls from the FILE thread.
-class ExternalPrefExtensionProvider : public StatefulExternalExtensionProvider {
- public:
-  explicit ExternalPrefExtensionProvider();
-  virtual ~ExternalPrefExtensionProvider();
-
-  // Used only during testing to not use the json file for external extensions,
-  // but instead parse a json file specified by the test.
-  void SetPreferencesForTesting(const std::string& json_data_for_testing);
-
- private:
-  void SetPreferences(ValueSerializer* serializer);
-};
-
-#endif  // CHROME_BROWSER_EXTENSIONS_EXTERNAL_PREF_EXTENSION_PROVIDER_H_