[ios] Fix reading list crash on shut down

Bug: 1108308
Change-Id: I1c0d7db1d5451f893736a9c59c9ec7a5b3b5edbd
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2351776Reviewed-by: Eric Noyau <noyau@chromium.org>
Commit-Queue: David Jean <djean@chromium.org>
Cr-Commit-Position: refs/heads/master@{#797179}

ios/chrome/browser/reading_list/offline_page_tab_helper.mm

@@ -224,12 +224,17 @@ void OfflinePageTabHelper::ReadingListModelLoaded(
 void OfflinePageTabHelper::ReadingListModelBeingDeleted(
     const ReadingListModel* model) {
   DCHECK(reading_list_model_ == nullptr || reading_list_model_ == model);
+
+  // Detach will nullify web_state_, this keeps it local a bit longer
+  // to allow removing user data below.
+  web::WebState* webState = web_state_;
+
   Detach();
 
   // The call to RemoveUserData cause the destruction of the current instance,
   // so nothing should be done after that point (this is like "delete this;").
   // Unregistration as an observer happens in the destructor.
-  web_state_->RemoveUserData(UserDataKey());
+  webState->RemoveUserData(UserDataKey());
 }
 
 void OfflinePageTabHelper::PresentOfflinePageForOnlineUrl(const GURL& url) {