implement user agent override option.

ios/web/public/test/fakes/test_navigation_manager.mm

 // Copyright 2016 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/test/fakes/test_navigation_manager.h"
 
 namespace web {
 
 TestNavigationManager::TestNavigationManager()
     : items_index_(-1),
@@ skipping 137 matching lines @@
 void TestNavigationManager::CopyStateFromAndPrune(
     const NavigationManager* source) {
   NOTREACHED();
 }
 
 bool TestNavigationManager::CanPruneAllButLastCommittedItem() const {
   NOTREACHED();
   return false;
 }
 
-void TestNavigationManager::OverrideDesktopUserAgentForNextPendingItem() {
-  NOTREACHED();
-}
-
 // Adds a new navigation item of |transition| type at the end of this
 // navigation manager.
 void TestNavigationManager::AddItem(const GURL& url,
                                     ui::PageTransition transition) {
   items_.push_back(web::NavigationItem::Create());
   items_.back()->SetTransitionType(transition);
   items_.back()->SetURL(url);
   SetLastCommittedItemIndex(GetItemCount() - 1);
 }
 
 }  // namespace web