Convert ios_web_shell to ARC.

ios/web/ios_web_shell_exe.gypi

 # Copyright 2016 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.
 
 {
   'type': 'executable',
   'mac_bundle': 1,
   'include_dirs': [
     '../..',
   ],
   'dependencies': [
     'ios_web.gyp:ios_web',
     'ios_web.gyp:ios_web_app',
     '../../base/base.gyp:base',
     '../../net/net.gyp:net',
     '../../net/net.gyp:net_extras',
     '../../ui/base/ui_base.gyp:ui_base',
   ],
   'export_dependent_settings': [
     'ios_web.gyp:ios_web',
     'ios_web.gyp:ios_web_app',
     '../../base/base.gyp:base',
     '../../net/net.gyp:net',
     '../../ui/base/ui_base.gyp:ui_base',
   ],
   'xcode_settings': {
     'INFOPLIST_FILE': 'shell/Info.plist',
+    'CLANG_ENABLE_OBJC_ARC': 'YES',
   },
   'sources': [
     'shell/app_delegate.h',
     'shell/app_delegate.mm',
     'shell/shell_browser_state.h',
     'shell/shell_browser_state.mm',
     'shell/shell_main_delegate.h',
     'shell/shell_main_delegate.mm',
     'shell/shell_network_delegate.cc',
     'shell/shell_network_delegate.h',
@@ skipping 16 matching lines @@
     ],
     'mac_bundle_resources': [
       'shell/Default.png',
       'shell/MainView.xib',
       'shell/textfield_background@2x.png',
       'shell/toolbar_back@2x.png',
       'shell/toolbar_forward@2x.png',
     ],
   },
 }