Use custom user data to serialize Tab last visited timestamp.

ios/web/public/crw_session_storage.mm

 // 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.
 
 #import "ios/web/public/crw_session_storage.h"
 
 #import "ios/web/navigation/crw_session_certificate_policy_manager.h"
 #import "ios/web/public/serializable_user_data_manager.h"
 
 #if !defined(__has_feature) || !__has_feature(objc_arc)
 #error "This file requires ARC support."
 #endif
 
 namespace {
 // Serialization keys used in NSCoding functions.
 NSString* const kCertificatePolicyManagerKey = @"certificatePolicyManager";
 NSString* const kCurrentNavigationIndexKey = @"currentNavigationIndex";
 NSString* const kItemStoragesKey = @"entries";
-NSString* const kLastVisitedTimestampKey = @"lastVisitedTimestamp";
 NSString* const kOpenedByDOMKey = @"openedByDOM";
 NSString* const kPreviousNavigationIndexKey = @"previousNavigationIndex";
 }
 
 @interface CRWSessionStorage () {
   // Backing object for property of same name.
   std::unique_ptr<web::SerializableUserData> _userData;
 }
 
 @end
 
 @implementation CRWSessionStorage
 
 @synthesize openedByDOM = _openedByDOM;
 @synthesize currentNavigationIndex = _currentNavigationIndex;
 @synthesize previousNavigationIndex = _previousNavigationIndex;
-@synthesize lastVisitedTimestamp = _lastVisitedTimestamp;
 @synthesize itemStorages = _itemStorages;
 @synthesize sessionCertificatePolicyManager = _sessionCertificatePolicyManager;
 
 #pragma mark - Accessors
 
 - (web::SerializableUserData*)userData {
   return _userData.get();
 }
 
 - (void)setSerializableUserData:
     (std::unique_ptr<web::SerializableUserData>)userData {
   _userData = std::move(userData);
 }
 
 #pragma mark - NSCoding
 
 - (instancetype)initWithCoder:(nonnull NSCoder*)decoder {
   self = [super init];
   if (self) {
     _openedByDOM = [decoder decodeBoolForKey:kOpenedByDOMKey];
     _currentNavigationIndex =
         [decoder decodeIntForKey:kCurrentNavigationIndexKey];
     _previousNavigationIndex =
         [decoder decodeIntForKey:kPreviousNavigationIndexKey];
-    _lastVisitedTimestamp =
-        [decoder decodeDoubleForKey:kLastVisitedTimestampKey];
     _itemStorages = [[NSMutableArray alloc]
         initWithArray:[decoder decodeObjectForKey:kItemStoragesKey]];
     // Prior to M34, 0 was used as "no index" instead of -1; adjust for that.
     if (!_itemStorages.count)
       _currentNavigationIndex = -1;
     _sessionCertificatePolicyManager =
         [decoder decodeObjectForKey:kCertificatePolicyManagerKey];
     if (!_sessionCertificatePolicyManager) {
       _sessionCertificatePolicyManager =
           [[CRWSessionCertificatePolicyManager alloc] init];
     }
     _userData = web::SerializableUserData::Create();
     _userData->Decode(decoder);
   }
   return self;
 }
 
 - (void)encodeWithCoder:(NSCoder*)coder {
   [coder encodeBool:self.openedByDOM forKey:kOpenedByDOMKey];
   [coder encodeInt:self.currentNavigationIndex
             forKey:kCurrentNavigationIndexKey];
   [coder encodeInt:self.previousNavigationIndex
             forKey:kPreviousNavigationIndexKey];
-  [coder encodeDouble:self.lastVisitedTimestamp
-               forKey:kLastVisitedTimestampKey];
   [coder encodeObject:self.itemStorages forKey:kItemStoragesKey];
   [coder encodeObject:self.sessionCertificatePolicyManager
                forKey:kCertificatePolicyManagerKey];
   if (_userData)
     _userData->Encode(coder);
 }
 
 @end