[builtins] move String.prototype[@@iterator] to C++ builtin

src/v8.gyp

 # Copyright 2012 the V8 project authors. All rights reserved.
 # Redistribution and use in source and binary forms, with or without
 # modification, are permitted provided that the following conditions are
 # met:
 #
 #     * Redistributions of source code must retain the above copyright
 #       notice, this list of conditions and the following disclaimer.
 #     * Redistributions in binary form must reproduce the above
 #       copyright notice, this list of conditions and the following
 #       disclaimer in the documentation and/or other materials provided
@@ skipping 2198 matching lines @@
           'js/regexp.js',
           'js/arraybuffer.js',
           'js/typedarray.js',
           'js/iterator-prototype.js',
           'js/collection.js',
           'js/weak-collection.js',
           'js/collection-iterator.js',
           'js/promise.js',
           'js/messages.js',
           'js/array-iterator.js',
-          'js/string-iterator.js',
           'js/templates.js',
           'js/spread.js',
           'js/proxy.js',
           'debug/mirrors.js',
           'debug/debug.js',
           'debug/liveedit.js',
         ],
         'experimental_library_files': [
           'js/macros.py',
           'messages.h',
@@ skipping 218 matching lines @@
       'conditions': [
         ['want_separate_host_toolset_mkpeephole==1', {
           'toolsets': ['host'],
         }, {
           'toolsets': ['target'],
         }],
       ],
     },
   ],
 }