Prevent error in file path

ios/chrome/browser/reading_list/url_downloader.h

 // Copyright 2016 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 IOS_CHROME_BROWSER_READING_LIST_URL_DOWNLOADER_H_
 #define IOS_CHROME_BROWSER_READING_LIST_URL_DOWNLOADER_H_
 
 #include <queue>
 
 #include "base/callback.h"
@@ skipping 95 matching lines @@
       const std::string& html,
       const std::vector<dom_distiller::DistillerViewerInterface::ImageInfo>&
           images);
   // Saves |html| to disk in the correct location for |url|; returns success.
   bool SaveHTMLForURL(std::string html, const GURL& url);
   // Downloads |url|, depending on |offlineURLExists| state.
   virtual void DownloadURL(GURL url, bool offlineURLExists);
   // Saves distilled html to disk, including saving images and main file.
   SuccessState SaveDistilledHTML(
       const GURL& url,
-      std::vector<dom_distiller::DistillerViewerInterface::ImageInfo> images,
-      std::string html);
+      const std::vector<dom_distiller::DistillerViewerInterface::ImageInfo>&
+          images,
+      const std::string& html);
   // Callback for distillation completion.
   void DistillerCallback(
       const GURL& pageURL,
       const std::string& html,
       const std::vector<dom_distiller::DistillerViewerInterface::ImageInfo>&
           images,
       const std::string& title);
 
   dom_distiller::DomDistillerService* distiller_service_;
   PrefService* pref_service_;
   const DownloadCompletion download_completion_;
   const SuccessCompletion delete_completion_;
   std::deque<Task> tasks_;
   bool working_;
   base::FilePath base_directory_;
   std::unique_ptr<dom_distiller::DistillerViewerInterface> distiller_;
   base::CancelableTaskTracker task_tracker_;
 
   DISALLOW_COPY_AND_ASSIGN(URLDownloader);
 };
 
 #endif  // IOS_CHROME_BROWSER_READING_LIST_URL_DOWNLOADER_H_