Revert of OOPIF: Fix subframe back/forward after recreating FTNs.

content/browser/frame_host/frame_navigation_entry.cc

 // Copyright 2015 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 "content/browser/frame_host/frame_navigation_entry.h"
 
 #include <utility>
 
 namespace content {
 
@@ skipping 41 matching lines @@
   item_sequence_number_ = item_sequence_number;
   document_sequence_number_ = document_sequence_number;
   site_instance_ = site_instance;
   url_ = url;
   referrer_ = referrer;
   page_state_ = page_state;
 }
 
 void FrameNavigationEntry::set_item_sequence_number(
     int64_t item_sequence_number) {
-  // TODO(creis): Assert that this does not change after being assigned, once
-  // location.replace is classified as NEW_PAGE rather than EXISTING_PAGE.
-  // Same for document sequence number.  See https://crbug.com/596707.
+  // Once assigned, the item sequence number shouldn't change.
+  DCHECK(item_sequence_number_ == -1 ||
+         item_sequence_number_ == item_sequence_number);
   item_sequence_number_ = item_sequence_number;
 }
 
 void FrameNavigationEntry::set_document_sequence_number(
     int64_t document_sequence_number) {
+  // Once assigned, the document sequence number shouldn't change.
+  DCHECK(document_sequence_number_ == -1 ||
+         document_sequence_number_ == document_sequence_number);
   document_sequence_number_ = document_sequence_number;
 }
 
 }  // namespace content