Create distiller files for Reading List.

ios/chrome/browser/reading_list/url_downloader_unittest.mm

Index: ios/chrome/browser/reading_list/url_downloader_unittest.mm
diff --git a/ios/chrome/browser/reading_list/url_downloader_unittest.mm b/ios/chrome/browser/reading_list/url_downloader_unittest.mm
index 717f208c01fa72a104344b358cb10e6b29ba35c6..b0bf2fc15a9104842482f0fde8387ef936042a54 100644
--- a/ios/chrome/browser/reading_list/url_downloader_unittest.mm
+++ b/ios/chrome/browser/reading_list/url_downloader_unittest.mm
@@ -40,8 +40,9 @@ class DistillerViewerTest : public dom_distiller::DistillerViewerInterface {
 class MockURLDownloader : public URLDownloader {
  public:
   MockURLDownloader(base::FilePath path)
-      : URLDownloader(nil,
-                      nil,
+      : URLDownloader(nullptr,
+                      nullptr,
+                      nullptr,
                       path,
                       base::Bind(&MockURLDownloader::OnEndDownload,
                                  base::Unretained(this)),