implement user agent override option.

ios/chrome/browser/tabs/tab.h

 // 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.
 
 #ifndef IOS_CHROME_BROWSER_TABS_TAB_H_
 #define IOS_CHROME_BROWSER_TABS_TAB_H_
 
 #import <UIKit/UIKit.h>
 
 #include <memory>
@@ skipping 267 matching lines @@
 // Returns the infobars::InfoBarManager object for this tab.
 - (infobars::InfoBarManager*)infoBarManager;
 
 // Whether the content of the current tab is compatible with reader mode.
 - (BOOL)canSwitchToReaderMode;
 
 // Asks the tab to enter into reader mode, presenting a streamlined view of the
 // current content.
 - (void)switchToReaderMode;
 
-// Update internal state to use the desktop user agent. Must call
-// -reloadWebViewAndURL for changes to take effect.
-- (void)enableDesktopUserAgent;
-
 // Remove the UIWebView and reload the current url.  Used by request desktop
 // so the updated user agent is used.
 - (void)reloadForDesktopUserAgent;
 
 // Ensures the toolbar visibility matches |visible|.
 - (void)updateFullscreenWithToolbarVisible:(BOOL)visible;
 
 // Returns a snapshot of the current page, backed by disk so it can be purged
 // and reloaded easily. The snapshot may be in memory, saved on disk or not
 // present at all.
@@ skipping 47 matching lines @@
 // Evaluates U2F result.
 - (void)evaluateU2FResultFromURL:(const GURL&)url;
 
 // Cancels prerendering. It is an error to call this on anything except a
 // prerender tab (where |isPrerenderTab| is set to YES).
 - (void)discardPrerender;
 
 @end
 
 #endif  // IOS_CHROME_BROWSER_TABS_TAB_H_