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

ios/chrome/browser/reading_list/url_downloader.h

Index: ios/chrome/browser/reading_list/url_downloader.h
diff --git a/ios/chrome/browser/reading_list/url_downloader.h b/ios/chrome/browser/reading_list/url_downloader.h
index 5ed50b283a05e1620e912c6813e931d3925caf5f..d8c9047377cd42da33b5e23a4ad57cd13211b0ac 100644
--- a/ios/chrome/browser/reading_list/url_downloader.h
+++ b/ios/chrome/browser/reading_list/url_downloader.h
@@ -109,6 +109,8 @@ class URLDownloader {
                  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,