Convert ios_web_view_shell to pure Objective-C.

ios/web_view/shell/shell_exe_main.m

 // 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]));
   }
 }