Add gn implementation of //ios/web/shell:ios_web_shell.

ios/web/ios_web.gyp

 # 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.
 
 {
   'variables': {
     'chromium_code': 1,
    },
   'targets': [
     {
+      # GN version: //ios/web/public/app
       'target_name': 'ios_web_app',
       'type': 'static_library',
       'include_dirs': [
         '../..',
       ],
       'dependencies': [
         'ios_web',
         '../../base/base.gyp:base',
         '../../base/base.gyp:base_i18n',
         '../../crypto/crypto.gyp:crypto',
@@ skipping 471 matching lines @@
       'dependencies': [
         '../../base/base.gyp:base'
       ],
       'sources': [
         'public/user_agent.h',
         'public/user_agent.mm',
       ],
     },
   ],
 }