Re-land "Replace requirePageReconstruction with setCustomUserAgent"

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 1359 matching lines @@
 - (BOOL)usesDesktopUserAgent {
   if (!self.navigationManager)
     return NO;
 
   web::NavigationItem* visibleItem = self.navigationManager->GetVisibleItem();
   return visibleItem &&
          visibleItem->GetUserAgentType() == web::UserAgentType::DESKTOP;
 }
 
 - (void)reloadWithUserAgentType:(web::UserAgentType)userAgentType {
-  // This removes the web view, which will be recreated at the end of this.
-  [self.webController requirePageReconstruction];
-
   // TODO(crbug.com/228171): A hack in session_controller -addPendingItem
   // discusses making tab responsible for distinguishing history stack
   // navigation from new navigations.
   web::NavigationManager* navigationManager = [self navigationManager];
   DCHECK(navigationManager);
 
   web::NavigationItem* lastNonRedirectItem =
       navigationManager->GetTransientItem();
   if (!lastNonRedirectItem || IsItemRedirectItem(lastNonRedirectItem))
     lastNonRedirectItem = navigationManager->GetVisibleItem();
@@ skipping 590 matching lines @@
 
 - (TabModel*)parentTabModel {
   return parentTabModel_;
 }
 
 - (FormInputAccessoryViewController*)inputAccessoryViewController {
   return inputAccessoryViewController_.get();
 }
 
 @end