Use custom user data to serialize Tab last visited timestamp.

ios/web/navigation/crw_session_controller.h

 // 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.
 
 #ifndef IOS_WEB_NAVIGATION_CRW_SESSION_CONTROLLER_H_
 #define IOS_WEB_NAVIGATION_CRW_SESSION_CONTROLLER_H_
 
 #import <Foundation/Foundation.h>
 #include <vector>
 
@@ skipping 23 matching lines @@
 @property(nonatomic, readonly, assign) NSInteger currentNavigationIndex;
 @property(nonatomic, readonly, assign) NSInteger previousNavigationIndex;
 // The index of the pending item if it is in |items|, or -1 if |pendingItem|
 // corresponds with a new navigation (created by addPendingItem:).
 @property(nonatomic, readwrite, assign) NSInteger pendingItemIndex;
 // Indicates whether the page was opened by DOM (e.g. with |window.open|
 // JavaScript call or by clicking a link with |_blank| target).
 @property(nonatomic, readonly, getter=isOpenedByDOM) BOOL openedByDOM;
 @property(nonatomic, readonly, strong)
     CRWSessionCertificatePolicyManager* sessionCertificatePolicyManager;
-@property(nonatomic, assign) NSTimeInterval lastVisitedTimestamp;
 
 // The list of CRWSessionEntries in |_entries|'s NavigationItemImpls.
 @property(nonatomic, readonly) web::NavigationItemList items;
 // The current NavigationItem.  During a pending navigation, returns the
 // NavigationItem for that navigation.  If a transient NavigationItem exists,
 // this NavigationItem will be returned.
 @property(nonatomic, readonly) web::NavigationItemImpl* currentItem;
 // Returns the NavigationItem whose URL should be displayed to the user.
 @property(nonatomic, readonly) web::NavigationItemImpl* visibleItem;
 // Returns the NavigationItem corresponding to a load for which no data has yet
@@ skipping 94 matching lines @@
 // same-document navigation.  Entries can be passed in any order.
 - (BOOL)isSameDocumentNavigationBetweenItem:(web::NavigationItem*)firstItem
                                     andItem:(web::NavigationItem*)secondItem;
 
 // Returns the index of |item| in |items|.
 - (NSInteger)indexOfItem:(const web::NavigationItem*)item;
 
 @end
 
 #endif  // IOS_WEB_NAVIGATION_CRW_SESSION_CONTROLLER_H_