Teach update_from_chromium.py to preserve certain files through roll.

mojo/tools/roll/utils.py

Index: mojo/tools/roll/utils.py
diff --git a/mojo/tools/roll/utils.py b/mojo/tools/roll/utils.py
index dcdfc3510e700aa7eace92f09761c7633986c533..bf10cdcbce014d03303eae1c0d5392c17f164ee3 100755
--- a/mojo/tools/roll/utils.py
+++ b/mojo/tools/roll/utils.py
@@ -3,9 +3,12 @@
 # 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)
@@ -23,6 +26,16 @@ def find(patterns, start='.'):
         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()