Format GN files and add a presubmit check that GN is properly formatted

pkg/BUILD.gn

 # Copyright (c) 2016, the Dart project authors.  Please see the AUTHORS file
 # for details. All rights reserved. Use of this source code is governed by a
 # BSD-style license that can be found in the LICENSE file.
 
 import("../utils/create_timestamp.gni")
 
 template("make_third_party_pkg_files_stamp") {
   assert(defined(invoker.id), "Must define the stamp file id")
   path = rebase_path("../third_party/pkg")
   if (defined(invoker.path)) {
@@ skipping 36 matching lines @@
     ":make_third_party_pkg_files_1_stamp",
     ":make_third_party_pkg_files_2_stamp",
     ":make_third_party_pkg_files_3_stamp",
   ]
 
   stamp0_outputs = get_target_outputs(":make_third_party_pkg_files_0_stamp")
   stamp1_outputs = get_target_outputs(":make_third_party_pkg_files_1_stamp")
   stamp2_outputs = get_target_outputs(":make_third_party_pkg_files_2_stamp")
   stamp3_outputs = get_target_outputs(":make_third_party_pkg_files_3_stamp")
 
-  inputs = stamp0_outputs +
-           stamp1_outputs +
-           stamp2_outputs +
-           stamp3_outputs
+  inputs = stamp0_outputs + stamp1_outputs + stamp2_outputs + stamp3_outputs
 
   outputs = [
-    "$root_gen_dir/pkg_files.stamp"
+    "$root_gen_dir/pkg_files.stamp",
   ]
 
   script = "../tools/create_timestamp_file.py"
-  args = [
-    rebase_path("$root_gen_dir/pkg_files.stamp"),
-  ]
+  args = [ rebase_path("$root_gen_dir/pkg_files.stamp") ]
 }