[CRD iOS] Refactor an interface for RemotingAuthorization

remoting/ios/app/remoting_menu_view_controller.mm

 // Copyright 2017 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.
 
 #if !defined(__has_feature) || !__has_feature(objc_arc)
 #error "This file requires ARC support."
 #endif
 
 #import "remoting/ios/app/remoting_menu_view_controller.h"
 
@@ skipping 185 matching lines @@
 }
 
 - (void)didTapGetAccessCode:(id)sender {
   NSString* authUri =
       [NSString stringWithCString:GetAuthorizationCodeUri().c_str()
                          encoding:[NSString defaultCStringEncoding]];
   [[UIApplication sharedApplication] openURL:[NSURL URLWithString:authUri]];
 }
 
 - (void)didTapLogout:(id)sender {
-  [[RemotingService SharedInstance].authentication logout];
+  [RemotingService.instance.authentication logout];
 }
 
 @end