Implement fetching, caching and retrieval of external policy data

chrome/browser/policy/external_data_fetcher.h

 // Copyright 2013 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_POLICY_EXTERNAL_DATA_FETCHER_H_
 #define CHROME_BROWSER_POLICY_EXTERNAL_DATA_FETCHER_H_
 
 #include <string>
 
 #include "base/callback_forward.h"
@@ skipping 26 matching lines @@
   // |callback| is invoked with a NULL pointer. Otherwise, the |callback| is
   // invoked with the referenced data once it has been successfully retrieved.
   // If retrieval is temporarily impossible (e.g. no network connectivity), the
   // |callback| will be invoked when the temporary hindrance is resolved. If
   // retrieval is permanently impossible (e.g. |policy_| references data that
   // does not exist on the server), the |callback| will never be invoked.
   void Fetch(const FetchCallback& callback) const;
 
  private:
   base::WeakPtr<ExternalDataManager> manager_;
-  std::string policy_;
+  const std::string policy_;
 };
 
 }  // namespace policy
 
 #endif  // CHROME_BROWSER_POLICY_EXTERNAL_DATA_FETCHER_H_