[iOS] Tentative fix for crash

This CL fixes a crash where the navigationItem associated with a
BackForwardListItem is nil on a back/forward navigation, leading
to a null pointer crash.
If the associated item is nil, the code path is now the same as a
non-back/forwarad navigation.

Fixed: 1121950
Change-Id: Ic4b66af2dc9aa368ff046d66095cf090591eed57
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2379724
Commit-Queue: Eugene But <eugenebut@chromium.org>
Reviewed-by: Ali Juma <ajuma@chromium.org>
Reviewed-by: Justin Cohen <justincohen@chromium.org>
Cr-Commit-Position: refs/heads/master@{#802824}

ios/web/navigation/crw_wk_navigation_handler.mm

@@ -1289,8 +1289,12 @@ void ReportOutOfSyncURLInDidStartProvisionalNavigation(
     item = [[CRWNavigationItemHolder
         holderForBackForwardListItem:webView.backForwardList.currentItem]
         navigationItem];
-    userAgentType = item->GetUserAgentType();
-  } else {
+    // In some cases, the associated item isn't found. In that case, follow the
+    // code path for the non-backforward navigations. See crbug.com/1121950.
+    if (item)
+      userAgentType = item->GetUserAgentType();
+  }
+  if (!item) {
     // Get the visible item. There is no guarantee that the pending item belongs
     // to this navigation but it is very likely that it is the case. If there is
     // no pending item, it is probably a render initiated navigation. Use the