Add DCHECKs to StatefulExternalExtensionProvider

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_