Teach update_from_chromium.py to preserve certain files through roll.

mojo/tools/roll/update_from_chromium.py

Index: mojo/tools/roll/update_from_chromium.py
diff --git a/mojo/tools/roll/update_from_chromium.py b/mojo/tools/roll/update_from_chromium.py
index 5d2bd4475f757e725a77eb6a82e80bfa4701f4d2..c4d5eacac2a9ca3f8d859449afc08e29be3e9bab 100755
--- a/mojo/tools/roll/update_from_chromium.py
+++ b/mojo/tools/roll/update_from_chromium.py
@@ -12,6 +12,7 @@ import urllib2
 from utils import commit
 from utils import mojo_root_dir
 from utils import system
+from utils import temp_dir
 
 import patch
 
@@ -88,6 +89,10 @@ net_dirs = [
 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):
@@ -123,8 +128,20 @@ def main():
       "snapshot of things imported from chromium.")
   parser.add_argument("chromium_dir", help="chromium source dir")
   args = parser.parse_args()
-  rev(args.chromium_dir)
-  patch.patch()
+  with temp_dir() as tempd:
+    for f in files_not_to_roll:
+      system(["mkdir", "-p", os.path.dirname(f)], cwd=tempd)
+      system(["cp", os.path.join(mojo_root_dir, f), os.path.join(tempd, f)])
+
+    rev(args.chromium_dir)
+    patch.patch()
+
+    print "Restoring files whose contents don't track Chromium"
+    for f in files_not_to_roll:
+      system(["mkdir", "-p", os.path.dirname(f)], cwd=mojo_root_dir)
+      system(["cp", os.path.join(tempd, f), os.path.join(mojo_root_dir, f)])
+    system(["git", "add", "."], cwd=mojo_root_dir)
+    commit("Restored pre-roll versions of files that don't get rolled")
   return 0
 
 if __name__ == "__main__":