[esnext] implement String padding proposal

BUILD.gn

 # Copyright 2014 The Chromium Authors. All rights reserved.
 # Use of this source code is governed by a BSD-style license that can be
 # found in the LICENSE file.
 
 import("//build/config/android/config.gni")
 import("//build/config/arm.gni")
 import("//build/config/mips.gni")
 import("//build/config/sanitizers/sanitizers.gni")
 
 if (is_android) {
@@ skipping 292 matching lines @@
     "src/js/macros.py",
     "src/messages.h",
     "src/js/generator.js",
     "src/js/harmony-atomics.js",
     "src/js/harmony-regexp.js",
     "src/js/harmony-object-observe.js",
     "src/js/harmony-sharedarraybuffer.js",
     "src/js/harmony-simd.js",
     "src/js/harmony-species.js",
     "src/js/harmony-unicode-regexps.js",
+    "src/js/harmony-string-padding.js",
     "src/js/promise-extra.js"
   ]
 
   outputs = [
     "$target_gen_dir/experimental-libraries.cc",
   ]
 
   args = [
            rebase_path("$target_gen_dir/experimental-libraries.cc",
                        root_build_dir),
@@ skipping 1771 matching lines @@
 
   configs -= [ "//build/config/compiler:chromium_code" ]
   configs += [ "//build/config/compiler:no_chromium_code" ]
   configs += [
     ":internal_config",
     ":libplatform_config",
     ":features",
     ":toolchain",
   ]
 }