Revert of Move GN grit scripts out of secondary tree.

tools/grit/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.
-
-# This target creates a stamp file that depends on all the sources in the grit
-# directory. By depending on this, a target can force itself to be rebuilt if
-# grit itself changes.
-action("grit_sources") {
-  depfile = "$target_out_dir/grit_sources.d"
-  script = "//tools/grit/stamp_grit_sources.py"
-
-  inputs = [
-    "grit.py",
-  ]
-
-  # Note that we can't call this "grit_sources.stamp" because that file is
-  # implicitly created by GN for script actions.
-  outputs = [
-    "$target_out_dir/grit_sources.script.stamp",
-  ]
-
-  args = [
-    rebase_path("//tools/grit", root_build_dir),
-    rebase_path(outputs[0], root_build_dir),
-    rebase_path(depfile, root_build_dir),
-  ]
-}