Remove all ReadingList entries on managed account signout.

ios/chrome/browser/reading_list/reading_list_download_service.cc

Index: ios/chrome/browser/reading_list/reading_list_download_service.cc
diff --git a/ios/chrome/browser/reading_list/reading_list_download_service.cc b/ios/chrome/browser/reading_list/reading_list_download_service.cc
index fba65f1f842c8b8862c08db969be96290b00f251..59286fb1c9840d9cbdf13f4a5f793fceeb341f17 100644
--- a/ios/chrome/browser/reading_list/reading_list_download_service.cc
+++ b/ios/chrome/browser/reading_list/reading_list_download_service.cc
@@ -112,6 +112,10 @@ void ReadingListDownloadService::ReadingListDidMoveEntry(
   ProcessNewEntry(url);
 }
 
+void ReadingListDownloadService::Clear() {
+  distiller_page_factory_->ReleaseAllRetainedWebState();
+}
+
 void ReadingListDownloadService::ProcessNewEntry(const GURL& url) {
   const ReadingListEntry* entry = reading_list_model_->GetEntryByURL(url);
   if (!entry || entry->IsRead()) {