Reland of (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 1532 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();
+- (BOOL)usesDesktopUserAgent {
+  if (!self.navigationManager)
+    return NO;
+
+  web::NavigationItem* visibleItem = self.navigationManager->GetVisibleItem();
+  return visibleItem && visibleItem->IsOverridingUserAgent();
 }
 
 - (void)enableDesktopUserAgent {
-  DCHECK_EQ(self.useDesktopUserAgent, NO);
+  DCHECK_EQ(self.usesDesktopUserAgent, NO);
   DCHECK([self navigationManager]);
   [self navigationManager]->OverrideDesktopUserAgentForNextPendingItem();
 }
 
 - (void)reloadForDesktopUserAgent {
   // |loadWithParams| will recreate the removed UIWebView.
   [self.webController requirePageReconstruction];
 
   // TODO(crbug.com/228171): A hack in session_controller -addPendingItem
   // discusses making tab responsible for distinguishing history stack
@@ skipping 636 matching lines @@
 
 - (TabModel*)parentTabModel {
   return parentTabModel_;
 }
 
 - (FormInputAccessoryViewController*)inputAccessoryViewController {
   return inputAccessoryViewController_.get();
 }
 
 @end