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

src/v8.gyp

Index: src/v8.gyp
diff --git a/src/v8.gyp b/src/v8.gyp
index 2c85310e49cff17dc621c5a85f0164bd6abc830b..5d99caba5ad9289fd3b23ee7307eef738dedc41f 100644
--- a/src/v8.gyp
+++ b/src/v8.gyp
@@ -502,6 +502,7 @@
         'builtins/builtins-handler.cc',
         'builtins/builtins-internal.cc',
         'builtins/builtins-interpreter.cc',
+        'builtins/builtins-iterator.cc',
         'builtins/builtins-json.cc',
         'builtins/builtins-math.cc',
         'builtins/builtins-number.cc',
@@ -2209,14 +2210,12 @@
           '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',