[iOS] Refactoring web CRWSessionController user agent code.

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 1515 matching lines @@
 }
 
 - (BOOL)useDesktopUserAgent {
   web::NavigationItem* currentItem = self.currentNavigationItem;
   return currentItem && currentItem->IsOverridingUserAgent();
 }
 
 - (void)enableDesktopUserAgent {
   DCHECK_EQ(self.useDesktopUserAgent, NO);
   DCHECK([self navigationManager]);
-  [[self navigationManager]->GetSessionController()
-          useDesktopUserAgentForNextPendingItem];
+  [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
   // navigation from new navigations. Because we want a new navigation here, we
   // use |PAGE_TRANSITION_FORM_SUBMIT|. When session_controller changes, so
@@ skipping 663 matching lines @@
 
 - (TabModel*)parentTabModel {
   return parentTabModel_;
 }
 
 - (FormInputAccessoryViewController*)inputAccessoryViewController {
   return inputAccessoryViewController_.get();
 }
 
 @end