[ReadingList iOS] Do not store HTTP images when distilling HTTPS pages

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 91 matching lines @@
   // the directory already exists.
   bool CreateOfflineURLDirectory(const GURL& url);
   // Saves the |data| for image at |imageURL| to disk, for main URL |url|;
   // puts path of saved file in |path| and returns whether save was successful.
   bool SaveImage(const GURL& url,
                  const GURL& imageURL,
                  const std::string& data,
                  std::string* image_name);
   // Saves images in |images| array to disk and replaces references in |html| to
   // local path. Returns updated html.
+  // If some images could not be saved, returns an empty string. It is the
+  // responsibility of the caller to clean the partial processing.
   std::string SaveAndReplaceImagesInHTML(
       const GURL& url,
       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.
@@ skipping 22 matching lines @@
   bool working_;
   base::FilePath base_directory_;
   GURL distilled_url_;
   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_