Fix regression with back-button not working on prerendered and instant pages.

content/browser/tab_contents/tab_contents.h

 // Copyright (c) 2011 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.
 
 #ifndef CONTENT_BROWSER_TAB_CONTENTS_TAB_CONTENTS_H_
 #define CONTENT_BROWSER_TAB_CONTENTS_TAB_CONTENTS_H_
 #pragma once
 
 #include <deque>
 #include <map>
@@ skipping 605 matching lines @@
   // or the dedicated set title message. It returns true if the new title is
   // different and was therefore updated.
   bool UpdateTitleForEntry(NavigationEntry* entry, const string16& title);
 
   // Causes the TabContents to navigate in the right renderer to |entry|, which
   // must be already part of the entries in the navigation controller.
   // This does not change the NavigationController state.
   bool NavigateToEntry(const NavigationEntry& entry,
                        NavigationController::ReloadType reload_type);
 
+  // Sets the history for this tab_contents to |history_length| entries, and
+  // moves the current page_id to the last entry in the list if it's valid.
+  // This is mainly used when a prerendered page is swapped into the current
+  // tab.
+  void SetHistoryLengthAndClear(int history_length);
+
+
   // Misc non-view stuff -------------------------------------------------------
 
   // Helper functions for sending notifications.
   void NotifySwapped();
   void NotifyConnected();
   void NotifyDisconnected();
 
   // RenderViewHostDelegate ----------------------------------------------------
 
   // RenderViewHostDelegate implementation.
@@ skipping 222 matching lines @@
   ObserverList<TabContentsObserver> observers_;
 
   // Content restrictions, used to disable print/copy etc based on content's
   // (full-page plugins for now only) permissions.
   int content_restrictions_;
 
   DISALLOW_COPY_AND_ASSIGN(TabContents);
 };
 
 #endif  // CONTENT_BROWSER_TAB_CONTENTS_TAB_CONTENTS_H_