ios: Properly handle nil restored_visible_items.

Bug: 1127434
Change-Id: I63c5db473c2987eebc3605206eff64aeb4db3490
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2461976
Commit-Queue: Justin Cohen <justincohen@chromium.org>
Commit-Queue: Gauthier Ambard <gambard@chromium.org>
Auto-Submit: Justin Cohen <justincohen@chromium.org>
Reviewed-by: Gauthier Ambard <gambard@chromium.org>
Cr-Commit-Position: refs/heads/master@{#815231}

ios/web/navigation/wk_based_navigation_manager_impl.mm

@@ -123,7 +123,7 @@ void WKBasedNavigationManagerImpl::OnNavigationStarted(const GURL& url) {
     // last_committed_web_view_item_ as the origins mistmatch.
     int index = GetLastCommittedItemIndexInCurrentOrRestoredSession();
     DCHECK(index != -1 || 0 == GetItemCount());
-    if (index != -1 &&
+    if (index != -1 && restored_visible_item_ &&
         restored_visible_item_->GetUserAgentType() != UserAgentType::NONE) {
       NavigationItemImpl* last_committed_item =
           GetNavigationItemImplAtIndex(static_cast<size_t>(index));
@@ -424,7 +424,8 @@ bool WKBasedNavigationManagerImpl::ShouldBlockUrlDuringRestore(
   // Abort restore.
   DiscardNonCommittedItems();
   last_committed_item_index_ = web_view_cache_.GetCurrentItemIndex();
-  if (restored_visible_item_->GetUserAgentType() != UserAgentType::NONE) {
+  if (restored_visible_item_ &&
+      restored_visible_item_->GetUserAgentType() != UserAgentType::NONE) {
     NavigationItem* last_committed_item =
         GetLastCommittedItemInCurrentOrRestoredSession();
     last_committed_item->SetUserAgentType(