Teach update_from_chromium.py to preserve certain files through roll.

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
@@ skipping 70 matching lines @@
 # These directories are temporarily cloned in order to support the network
 # bootstrap library until we get that sorted out.
 net_dirs = [
     "crypto",
     "net",
     "mojo/services/network"]
 
 files_to_copy = ["sandbox/sandbox_export.h",
     ".clang-format"]
 
+# The contents of these files before the roll will be preserved after the roll,
+# even though they live in directories rolled in from Chromium.
+files_not_to_roll = [ "build/config/mojo.gni" ]
+
 dirs = dirs_to_snapshot + net_dirs
 
 def chromium_rev_number(src_commit):
   base_url = "https://cr-rev.appspot.com/_ah/api/crrev/v1/commit/"
   commit_info = json.load(urllib2.urlopen(base_url + src_commit))
   return commit_info["numberings"][0]["number"]
 
 def rev(source_dir):
   for d in dirs:
     print "removing directory %s" % d
@@ skipping 15 matching lines @@
   system(["git", "add", "."], cwd=mojo_root_dir)
   src_commit = system(["git", "rev-parse", "HEAD"], cwd=source_dir).strip()
   src_rev = chromium_rev_number(src_commit)
   commit("Update from https://crrev.com/" + src_rev, cwd=mojo_root_dir)
 
 def main():
   parser = argparse.ArgumentParser(description="Update the mojo repo's " +
       "snapshot of things imported from chromium.")
   parser.add_argument("chromium_dir", help="chromium source dir")
   args = parser.parse_args()
+  pre_roll_commit = system(
+      ["git", "rev-parse", "HEAD"], cwd=mojo_root_dir).strip()
+
   rev(args.chromium_dir)
   patch.patch()
+
+  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())