Use a joinable background thread instead of worker pool in gin

mojo/tools/roll/update_from_chromium.py

 #!/usr/bin/env python
 # 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 argparse
 import json
 import os
 import subprocess
 import sys
 import urllib2
 from utils import commit
 from utils import mojo_root_dir
 from utils import system
 
 import patch
 
 # These directories are snapshotted from chromium without modifications.
 dirs_to_snapshot = [
     "base",
     "build",
     "cc",
-    "gin",
     "gpu",
     "sandbox/linux",
     "sdch",
     "skia",
     "testing",
     "third_party/android_testrunner",
     "third_party/ashmem",
     "third_party/binutils",
     "third_party/boringssl",
     "third_party/brotli",
@@ skipping 95 matching lines @@
 
   print "Restoring files whose contents don't track Chromium"
   for f in files_not_to_roll:
     system(["git", "checkout", pre_roll_commit, "--", f], cwd=mojo_root_dir)
   if files_not_to_roll:
     commit("Restored pre-roll versions of files that don't get rolled")
   return 0
 
 if __name__ == "__main__":
   sys.exit(main())