Store NavigationItem's unique ID in navigation context.

ios/web/web_state/navigation_context_impl.h

 // 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.
 
 #ifndef IOS_WEB_WEB_STATE_NAVIGATION_CONTEXT_IMPL_H_
 #define IOS_WEB_WEB_STATE_NAVIGATION_CONTEXT_IMPL_H_
 
 #include <memory>
 
 #include "base/macros.h"
@@ skipping 24 matching lines @@
   bool IsErrorPage() const override;
   net::HttpResponseHeaders* GetResponseHeaders() const override;
   ~NavigationContextImpl() override;
 
   // Setters for navigation context data members.
   void SetIsSameDocument(bool is_same_document);
   void SetIsErrorPage(bool is_error_page);
   void SetResponseHeaders(
       const scoped_refptr<net::HttpResponseHeaders>& response_headers);
 
+  // Optional unique id of the navigation item associated with this navigaiton.
+  int GetNavigationItemUniqueID() const;
+  void SetNavigationItemUniqueID(int unique_id);
+
  private:
   NavigationContextImpl(
       WebState* web_state,
       const GURL& url,
       bool is_same_page,
       bool is_error_page,
       const scoped_refptr<net::HttpResponseHeaders>& response_headers);
 
   WebState* web_state_ = nullptr;
   GURL url_;
   bool is_same_document_ = false;
   bool is_error_page_ = false;
   scoped_refptr<net::HttpResponseHeaders> response_headers_;
+  int navigation_item_unique_id_ = -1;
 
   DISALLOW_COPY_AND_ASSIGN(NavigationContextImpl);
 };
 
 }  // namespace web
 
 #endif  // IOS_WEB_WEB_STATE_NAVIGATION_CONTEXT_IMPL_H_