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

pkg/BUILD.gn

Index: pkg/BUILD.gn
diff --git a/pkg/BUILD.gn b/pkg/BUILD.gn
index 8dc602f35b585c5edcaf6672b513c7c72318ba75..e1ce11d2e4e545eee9792b14a469504a329f4ef0 100644
--- a/pkg/BUILD.gn
+++ b/pkg/BUILD.gn
@@ -54,17 +54,12 @@ action("pkg_files_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") ]
 }