add DCHECK that a server redirect pending item should never be added.

ios/web/navigation/crw_session_controller.mm

Index: ios/web/navigation/crw_session_controller.mm
diff --git a/ios/web/navigation/crw_session_controller.mm b/ios/web/navigation/crw_session_controller.mm
index ec96f47dcabb4f70455b788b5610428df4e61e56..2fcda1a4ed010e0e7a8dc1bf51b4afedd66217bf 100644
--- a/ios/web/navigation/crw_session_controller.mm
+++ b/ios/web/navigation/crw_session_controller.mm
@@ -285,6 +285,10 @@ - (void)addPendingItem:(const GURL&)url
               referrer:(const web::Referrer&)ref
             transition:(ui::PageTransition)trans
         initiationType:(web::NavigationInitiationType)initiationType {
+  // Server side redirects are handled by updating existing pending item instead
+  // of adding a new item.
+  DCHECK((trans & ui::PAGE_TRANSITION_SERVER_REDIRECT) == 0);
+
   [self discardTransientItem];
   self.pendingItemIndex = -1;