Convert ios_web_view_shell to pure Objective-C.

ios/web_view/shell/shell_exe_main.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 <UIKit/UIKit.h>
-
-#import "ios/web_view/shell/shell_app_delegate.h"
-
-#if !defined(__has_feature) || !__has_feature(objc_arc)
-#error "This file requires ARC support."
-#endif
-
-int main(int argc, char* argv[]) {
-  @autoreleasepool {
-    return UIApplicationMain(argc, argv, nil,
-                             NSStringFromClass([ShellAppDelegate class]));
-  }
-}