ios: Fix crash in RestoreItemsState.

GetNavigationItemImplAtIndex() appears to return a nil |cached_item|
sometimes, perhaps due to a mismatch in WKWebView's backForwardList.
Returning early when this happens in RestoreItemsState() may break
some restore state features, but should not put the user in a broken
state.

Bug: 1134096
Change-Id: I47a2f1316375860b223fa988e7aca9e4557057d0
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2454291Reviewed-by: Ali Juma <ajuma@chromium.org>
Reviewed-by: Gauthier Ambard <gambard@chromium.org>
Commit-Queue: Justin Cohen <justincohen@chromium.org>
Cr-Commit-Position: refs/heads/master@{#815204}

ios/web/navigation/wk_based_navigation_manager_impl.mm

@@ -794,6 +794,13 @@ void WKBasedNavigationManagerImpl::RestoreItemsState(
             cache_index, true /* create_if_missing */);
     NavigationItem* restore_item = items_restored[index].get();
 
+    // |cached_item| appears to be nil sometimes, perhaps due to a mismatch in
+    // WKWebView's backForwardList.  Returning early here may break some restore
+    // state features, but should not put the user in a broken state.
+    if (!cached_item || !restore_item) {
+      continue;
+    }
+
     bool is_same_url = cached_item->GetURL() == restore_item->GetURL();
     if (wk_navigation_util::IsRestoreSessionUrl(cached_item->GetURL())) {
       GURL target_url;