Store NavigationItem's unique ID in navigation context.

ios/web/web_state/navigation_context_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.
 
 #include "ios/web/web_state/navigation_context_impl.h"
 
 #import <Foundation/Foundation.h>
 
 #include "base/memory/ptr_util.h"
 #include "net/http/http_response_headers.h"
@@ skipping 47 matching lines @@
 
 void NavigationContextImpl::SetIsErrorPage(bool is_error_page) {
   is_error_page_ = is_error_page;
 }
 
 void NavigationContextImpl::SetResponseHeaders(
     const scoped_refptr<net::HttpResponseHeaders>& response_headers) {
   response_headers_ = response_headers;
 }
 
+int NavigationContextImpl::GetNavigationItemUniqueID() const {
+  return navigation_item_unique_id_;
+}
+
+void NavigationContextImpl::SetNavigationItemUniqueID(int unique_id) {
+  navigation_item_unique_id_ = unique_id;
+}
+
 NavigationContextImpl::NavigationContextImpl(
     WebState* web_state,
     const GURL& url,
     bool is_same_document,
     bool is_error_page,
     const scoped_refptr<net::HttpResponseHeaders>& response_headers)
     : web_state_(web_state),
       url_(url),
       is_same_document_(is_same_document),
       is_error_page_(is_error_page),
       response_headers_(response_headers) {}
 
 NavigationContextImpl::~NavigationContextImpl() = default;
 
 }  // namespace web