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

src/builtins/builtins-iterator.cc

 // Copyright 2016 the V8 project authors. All rights reserved.
 // Use of this source code is governed by a BSD-style license that can be
 // found in the LICENSE file.
 
-#ifndef V8_FILE_UTILS_H_
-#define V8_FILE_UTILS_H_
+#include "src/builtins/builtins.h"
+#include "src/builtins/builtins-utils.h"
 
 namespace v8 {
 namespace internal {
 
-// Helper functions to manipulate file paths.
-
-char* RelativePath(char** buffer, const char* exec_path, const char* name);
+void Builtins::Generate_IteratorPrototypeIterator(
+    CodeStubAssembler* assembler) {
+  assembler->Return(assembler->Parameter(0));
+}
 
 }  // namespace internal
 }  // namespace v8
-
-#endif  // V8_FILE_UTILS_H_