Migrate some case conversion functions from JS to CPP builtins

src/builtins/builtins-intl.cc

+// Copyright 2017 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.
+
+#ifdef V8_I18N_SUPPORT
+
+#include "src/builtins/builtins-utils.h"
+#include "src/builtins/builtins.h"
+#include "src/i18n.h"
+#include "src/objects-inl.h"
+
+namespace v8 {
+namespace internal {
+
+BUILTIN(StringPrototypeToLowerCaseI18N) {
+  HandleScope scope(isolate);
+  TO_THIS_STRING(string, "String.prototype.toLowerCase");
+  string = String::Flatten(string);
+  return ConvertCase(string, false, isolate);
+}
+
+BUILTIN(StringPrototypeToUpperCaseI18N) {
+  HandleScope scope(isolate);
+  TO_THIS_STRING(string, "String.prototype.toUpperCase");
+  string = String::Flatten(string);
+  return ConvertCase(string, true, isolate);
+}
+
+}  // namespace internal
+}  // namespace v8
+
+#endif  // V8_I18N_SUPPORT