[ios] Fixed GetIndexForOffset for pending transient items.

ios/web/navigation/navigation_manager_impl.mm

 // Copyright 2013 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.
 
 #include "ios/web/navigation/navigation_manager_impl.h"
 
 #include <stddef.h>
 
 #include <utility>
 
@@ skipping 326 matching lines @@
     NavigationManagerImpl* navigation_manager) {
   SetSessionController([navigation_manager->GetSessionController() copy]);
 }
 
 int NavigationManagerImpl::GetIndexForOffset(int offset) const {
   int result = [session_controller_ pendingEntryIndex] == -1
                    ? GetCurrentItemIndex()
                    : static_cast<int>([session_controller_ pendingEntryIndex]);
 
   if (offset < 0) {
-    if (GetTransientItem()) {
-      // Going back from transient item is a matter of discarding it and there
-      // is no need to move navigation index back.
+    if (GetTransientItem() && [session_controller_ pendingEntryIndex] == -1) {
+      // Going back from transient item that added to the end navigation stack
+      // is a matter of discarding it as there is no need to move navigation
+      // index back.
       offset++;
     }
 
     while (offset < 0 && result > 0) {
       // To stop the user getting 'stuck' on redirecting pages they weren't
       // even aware existed, it is necessary to pass over pages that would
       // immediately result in a redirect (the item *before* the redirected
       // page).
       while (result > 0 && IsRedirectItemAtIndex(result)) {
         --result;
@@ skipping 36 matching lines @@
 }
 
 bool NavigationManagerImpl::IsRedirectItemAtIndex(int index) const {
   DCHECK_GT(index, 0);
   DCHECK_LT(index, GetItemCount());
   ui::PageTransition transition = GetItemAtIndex(index)->GetTransitionType();
   return transition & ui::PAGE_TRANSITION_IS_REDIRECT_MASK;
 }
 
 }  // namespace web