Introduce git merge driver for the blink reformatting

clang_format_merge_driver.py

Index: clang_format_merge_driver.py
diff --git a/clang_format_merge_driver.py b/clang_format_merge_driver.py
new file mode 100755
index 0000000000000000000000000000000000000000..c40a85664b6980d20a045f57b33a9c185b943d96
--- /dev/null
+++ b/clang_format_merge_driver.py
@@ -0,0 +1,36 @@
+#!/usr/bin/env python
+# Copyright 2016 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.
+
+"""Clang-format 3-way merge driver.
+
+"""
+
+import subprocess
+import sys
+
+import clang_format
+
+def main():
+  if len(sys.argv) < 6:
+    print('usage: %s <base> <current> <others> <ignored> <path in the tree>' %
+        sys.argv[0])
+    sys.exit(1)
+
+  base, current, others, _, file_name_in_tree = sys.argv[1:6]
+  print '\nRunning clang-format 3-way merge driver on ' + file_name_in_tree
+
+  try:
+    tool = clang_format.FindClangFormatToolInChromiumTree()
+    for fpath in base, current, others:
+      subprocess.call([tool, '-i', '--style=chromium', fpath])
+  except clang_format.NotFoundError, e:
+    print e
+    print 'Failed to find clang-format. Falling-back on standard 3-way merge'
+
+  return subprocess.call(['git', 'merge-file', '-Lcurrent', '-Lbase', '-Lother',
+                          current, base, others])
+
+if __name__ == '__main__':
+  sys.exit(main())