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

BUILD.gn

Index: BUILD.gn
diff --git a/BUILD.gn b/BUILD.gn
index 1d02296654aab8a89c0e89bbabd1e94befdd950e..e9a431d66a47ff1cd84b2743da99fba803d465a7 100644
--- a/BUILD.gn
+++ b/BUILD.gn
@@ -398,14 +398,12 @@ action("js2c") {
     "src/js/regexp.js",
     "src/js/arraybuffer.js",
     "src/js/typedarray.js",
-    "src/js/iterator-prototype.js",
     "src/js/collection.js",
     "src/js/weak-collection.js",
     "src/js/collection-iterator.js",
     "src/js/promise.js",
     "src/js/messages.js",
     "src/js/array-iterator.js",
-    "src/js/string-iterator.js",
     "src/js/templates.js",
     "src/js/spread.js",
     "src/js/proxy.js",
@@ -931,6 +929,7 @@ v8_source_set("v8_base") {
     "src/builtins/builtins-handler.cc",
     "src/builtins/builtins-internal.cc",
     "src/builtins/builtins-interpreter.cc",
+    "src/builtins/builtins-iterator.cc",
     "src/builtins/builtins-json.cc",
     "src/builtins/builtins-math.cc",
     "src/builtins/builtins-number.cc",