Implement CryptAuthAccessTokenFetcher.

components/proximity_auth/cryptauth/cryptauth_access_token_fetcher.h

 // Copyright 2014 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 COMPONENTS_PROXIMITY_AUTH_CRYPT_AUTH_ACCESS_TOKEN_FETCHER_H
 #define COMPONENTS_PROXIMITY_AUTH_CRYPT_AUTH_ACCESS_TOKEN_FETCHER_H
 
+#include <string>
+
+#include "base/callback_forward.h"
+
 namespace proximity_auth {
 
 // Simple interface for fetching the OAuth2 access token that authorizes
-// CryptAuth API calls.
+// CryptAuth API calls. Do not reuse this after calling FetchAccessToken();
+// instead, create a new instance.
 class CryptAuthAccessTokenFetcher {
  public:
   virtual ~CryptAuthAccessTokenFetcher() {}
 
   // Fetches the access token asynchronously, invoking the callback upon
   // completion. If the fetch fails, the callback will be invoked with an empty
   // string.
   typedef base::Callback<void(const std::string&)> AccessTokenCallback;
   virtual void FetchAccessToken(const AccessTokenCallback& callback) = 0;
 };
 
 }  // namespace proximity_auth
 
 #endif  // COMPONENTS_PROXIMITY_AUTH_CRYPT_AUTH_ACCESS_TOKEN_FETCHER_H