Remove all ReadingList entries on managed account signout.

ios/chrome/browser/reading_list/url_downloader.cc

Index: ios/chrome/browser/reading_list/url_downloader.cc
diff --git a/ios/chrome/browser/reading_list/url_downloader.cc b/ios/chrome/browser/reading_list/url_downloader.cc
index 65963e6ad121a57f474b3d17b87b9a456b247293..0f58433b6dae4f3ae3430d94249ce943a3245d29 100644
--- a/ios/chrome/browser/reading_list/url_downloader.cc
+++ b/ios/chrome/browser/reading_list/url_downloader.cc
@@ -219,6 +219,11 @@ void URLDownloader::OnURLFetchComplete(const net::URLFetcher* source) {
                  base::Unretained(this), source->GetOriginalURL(), "", path));
 }
 
+void URLDownloader::CancelTask() {
+  task_tracker_.TryCancelAll();
+  distiller_.reset();
+}
+
 void URLDownloader::FetchPDFFile() {
   const GURL& pdf_url =
       distilled_url_.is_valid() ? distilled_url_ : original_url_;