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 550 matching lines @@
   void OnDidRunInsecureContent(const std::string& security_origin,
                                const GURL& target_url);
   void OnDocumentLoadedInFrame(int64 frame_id);
   void OnDidFinishLoad(int64 frame_id);
   void OnUpdateContentRestrictions(int restrictions);
   void OnGoToEntryAtOffset(int offset);
   void OnUpdateZoomLimits(int minimum_percent,
                           int maximum_percent,
                           bool remember);
   void OnFocusedNodeChanged(bool is_editable_node);
+  void OffsetAndPruneHistory(int offset);
 
   // Changes the IsLoading state and notifies delegate as needed
   // |details| is used to provide details on the load that just finished
   // (but can be null if not applicable). Can be overridden.
   void SetIsLoading(bool is_loading,
                     LoadNotificationDetails* details);
 
   // Called by derived classes to indicate that we're no longer waiting for a
   // response. This won't actually update the throbber, but it will get picked
   // up at the next animation step if the throbber is going.
@@ skipping 277 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_