Teach update_from_chromium.py to preserve certain files through roll.

mojo/tools/roll/utils.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 contextlib
 import fnmatch
 import os
 import subprocess
+import shutil
+import tempfile
 
 mojo_root_dir = os.path.join(os.path.dirname(os.path.realpath(__file__)),
                              os.pardir, os.pardir, os.pardir)
 
 def commit(message, cwd=None):
   subprocess.call(['git', 'commit', '-a', '-m', message], cwd=cwd)
 
 def system(command, cwd=None):
   return subprocess.check_output(command, cwd=cwd)
 
 def find(patterns, start='.'):
   for path, dirs, files in os.walk(start):
     for basename in files + dirs:
       if any([fnmatch.fnmatch(basename, pattern) for pattern in patterns]):
         filename = os.path.join(path, basename)
         yield filename
 
+# From
+# http://stackoverflow.com/questions/13379742/right-way-to-clean-up-a-temporary-folder-in-python-class
+@contextlib.contextmanager
+def temp_dir(*args, **kwargs):
+  d = tempfile.mkdtemp(*args, **kwargs)
+  try:
+    yield d
+  finally:
+    shutil.rmtree(d)
+
 def filter_file(path, predicate):
   with open(path, 'r+') as f:
     lines = f.readlines()
     new_lines = [line for line in lines if predicate(line)]
     f.seek(0)
     f.truncate()
     f.write(''.join(new_lines))