(Set)IsOverridingUserAgent should be called on VisibleItem

ios/chrome/browser/tabs/tab.mm

 // Copyright 2012 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/chrome/browser/tabs/tab.h"
 
 #import <CoreLocation/CoreLocation.h>
 #import <UIKit/UIKit.h>
 
 #include <utility>
@@ skipping 1550 matching lines @@
   [self.readerModeController exitReaderMode];
 }
 
 #pragma mark -
 
 - (void)openAppStore:(NSString*)appId {
   [storeKitLauncher_ openAppStore:appId];
 }
 
 - (BOOL)useDesktopUserAgent {
-  web::NavigationItem* currentItem = self.currentNavigationItem;
-  return currentItem && currentItem->IsOverridingUserAgent();
+  return [self navigationManager]->GetUseDesktopUserAgent();
 }
 
 - (void)enableDesktopUserAgent {
   DCHECK_EQ(self.useDesktopUserAgent, NO);
   DCHECK([self navigationManager]);
   [self navigationManager]->OverrideDesktopUserAgentForNextPendingItem();
 }
 
 - (void)reloadForDesktopUserAgent {
   // |loadWithParams| will recreate the removed UIWebView.
@@ skipping 640 matching lines @@
 
 - (TabModel*)parentTabModel {
   return parentTabModel_;
 }
 
 - (FormInputAccessoryViewController*)inputAccessoryViewController {
   return inputAccessoryViewController_.get();
 }
 
 @end