Use custom user data to serialize Tab last visited timestamp.

ios/web/navigation/serializable_user_data_manager_impl.mm

 // Copyright 2017 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/navigation/serializable_user_data_manager_impl.h"
 
 #import "base/mac/foundation_util.h"
 #import "ios/web/public/web_state/web_state.h"
 
 #if !defined(__has_feature) || !__has_feature(objc_arc)
@@ skipping 44 matching lines @@
 
 // static
 std::unique_ptr<SerializableUserData> SerializableUserData::Create() {
   return std::unique_ptr<SerializableUserData>(new SerializableUserDataImpl());
 }
 
 SerializableUserDataImpl::SerializableUserDataImpl()
     : data_(@{}), legacy_key_conversions_(@{
         @"tabId" : @"TabID",
         @"openerId" : @"OpenerID",
-        @"openerNavigationIndex" : @"OpenerNavigationIndex"
+        @"openerNavigationIndex" : @"OpenerNavigationIndex",
+        @"lastVisitedTimestamp" : @"LastVisitedTimestamp",
       }) {}
 
 SerializableUserDataImpl::~SerializableUserDataImpl() {}
 
 SerializableUserDataImpl::SerializableUserDataImpl(NSDictionary* data)
     : data_([data copy]) {}
 
 void SerializableUserDataImpl::Encode(NSCoder* coder) {
   [coder encodeObject:data_ forKey:kSerializedUserDataKey];
 }
@@ skipping 47 matching lines @@
 
 void SerializableUserDataManagerImpl::AddSerializableUserData(
     SerializableUserData* data) {
   DCHECK(data);
   SerializableUserDataImpl* data_impl =
       static_cast<SerializableUserDataImpl*>(data);
   data_.reset([data_impl->data() mutableCopy]);
 }
 
 }  // namespace web