Add a gn implementation for //ios/public/provider/web target.

ios/provider/ios_provider_web.gyp

 # Copyright 2013 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/public/provider/web
       'target_name': 'ios_provider_web',
       'type': 'static_library',
       'include_dirs': [
         '../..',
       ],
       'dependencies': [
         '../../base/base.gyp:base',
         '../web/ios_web.gyp:ios_web',
       ],
       'sources': [
         '../public/provider/web/web_controller_provider.h',
         '../public/provider/web/web_controller_provider.mm',
         '../public/provider/web/web_ui_ios.h',
         '../public/provider/web/web_ui_ios_controller.cc',
         '../public/provider/web/web_ui_ios_controller.h',
         '../public/provider/web/web_ui_ios_controller_factory.h',
         '../public/provider/web/web_ui_ios_message_handler.cc',
         '../public/provider/web/web_ui_ios_message_handler.h',
       ],
     },
   ],
 }