Clean up threading model of external extension providers

chrome/browser/extensions/external_policy_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_POLICY_EXTENSION_PROVIDER_H_
-#define CHROME_BROWSER_EXTENSIONS_EXTERNAL_POLICY_EXTENSION_PROVIDER_H_
-#pragma once
-
-#include "chrome/browser/extensions/stateful_external_extension_provider.h"
-
-class ListValue;
-class MockExternalPolicyExtensionProviderVisitor;
-class PrefService;
-
-// A specialization of the ExternalExtensionProvider that uses
-// prefs::kExtensionInstallForceList to look up which external extensions are
-// registered. The value of this preference is received via the constructor and
-// via |SetPreferences| in case of run-time updates.
-// 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 ExternalPolicyExtensionProvider
-    : public StatefulExternalExtensionProvider {
- public:
-  explicit ExternalPolicyExtensionProvider(const ListValue* forcelist);
-  virtual ~ExternalPolicyExtensionProvider();
-
-  // Set the internal list of extensions based on |forcelist|.
-  // Does not take ownership of |forcelist|.
-  void SetPreferences(const ListValue* forcelist);
-
- private:
-  friend class MockExternalPolicyExtensionProviderVisitor;
-
-  // Set the internal list of extensions based on |forcelist|.
-  // Does not take ownership of |forcelist|.
-  void ProcessPreferences(const ListValue* forcelist);
-};
-
-#endif  // CHROME_BROWSER_EXTENSIONS_EXTERNAL_POLICY_EXTENSION_PROVIDER_H_