Make SdchDictionaryFetcher free more memory when done.

net/url_request/sdch_dictionary_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.
 
 // TODO(rdsmith): This class needs to delegate URLRequest::Delegate methods
 // to the net/ embedder for correct implementation of authentication.
 // Specifically, this class needs the embedder to provide functionality
 // corresponding to
 // URLRequest::Delegate::{OnAuthRequired,OnCertificateRequested}.
 
@@ skipping 94 matching lines @@
 
   // A queue of URLs that are being used to download dictionaries.
   std::unique_ptr<UniqueFetchQueue> fetch_queue_;
 
   // The request, buffer, and consumer supplied data used for getting
   // the current dictionary.  All are null when a fetch is not in progress.
   std::unique_ptr<URLRequest> current_request_;
   scoped_refptr<IOBuffer> buffer_;
   OnDictionaryFetchedCallback current_callback_;
 
-  // The currently accumulating dictionary.
-  std::string dictionary_;
+  // The currently accumulating dictionary. Stored as a unique_ptr so all memory
+  // it consumes can be easily freed, as it gets quite big, and
+  // std::string::clear() may not free memory.
+  std::unique_ptr<std::string> dictionary_;
 
   // Store the URLRequestContext associated with the owning SdchManager for
   // use while fetching.
   URLRequestContext* const context_;
 
   DISALLOW_COPY_AND_ASSIGN(SdchDictionaryFetcher);
 };
 
 }  // namespace net
 
 #endif  // NET_URL_REQUEST_SDCH_DICTIONARY_FETCHER_H_