Convert ios_web_shell to ARC.

ios/web/shell/app_delegate.mm

 // Copyright 2014 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/web/shell/app_delegate.h"
 
 #include <memory>
 
 #import "base/mac/scoped_nsobject.h"
 #include "ios/web/public/app/web_main.h"
 #include "ios/web/public/web_client.h"
 #include "ios/web/public/web_state/web_state.h"
 #include "ios/web/shell/shell_browser_state.h"
 #include "ios/web/shell/shell_main_delegate.h"
 #include "ios/web/shell/shell_web_client.h"
 #import "ios/web/shell/view_controller.h"
 
+#if !defined(__has_feature) || !__has_feature(objc_arc)
+#error "This file requires ARC support."
+#endif
+
 @interface AppDelegate () {
   std::unique_ptr<web::ShellMainDelegate> _delegate;
   std::unique_ptr<web::WebMain> _webMain;
 }
 @end
 
 @implementation AppDelegate
 
 @synthesize window = _window;
 
@@ skipping 28 matching lines @@
 
 - (void)applicationDidBecomeActive:(UIApplication*)application {
 }
 
 - (void)applicationWillTerminate:(UIApplication*)application {
   _webMain.reset();
   _delegate.reset();
 }
 
 @end