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

ios/web/navigation/crw_session_controller.mm

 // Copyright 2012 The Chromium Authors. All rights reserved.
 // Use of this source code is governed by a BSD-style license that can be
 // found in the LICENSE file.
 
 #import "ios/web/navigation/crw_session_controller.h"
 
 #include <stddef.h>
 
 #include <algorithm>
 #include <utility>
@@ skipping 267 matching lines @@
 - (void)setBrowserState:(web::BrowserState*)browserState {
   _browserState = browserState;
   DCHECK(!_navigationManager ||
          _navigationManager->GetBrowserState() == _browserState);
 }
 
 - (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;
 
   // Don't create a new item if it's already the same as the current item,
   // allowing this routine to be called multiple times in a row without issue.
   // Note: CRWSessionController currently has the responsibility to distinguish
   // between new navigations and history stack navigation, hence the inclusion
   // of specific transiton type logic here, in order to make it reliable with
   // real-world observed behavior.
   // TODO(crbug.com/676129): Fix the way changes are detected/reported elsewhere
@@ skipping 323 matching lines @@
   return item;
 }
 
 - (BOOL)isRedirectTransitionForItemAtIndex:(size_t)index {
   DCHECK_LT(index, self.items.size());
   ui::PageTransition transition = self.items[index]->GetTransitionType();
   return (transition & ui::PAGE_TRANSITION_IS_REDIRECT_MASK) ? YES : NO;
 }
 
 @end