Add executable bit and shebang to python files

BUG=skia:
GOLD_TRYBOT_URL= https://gold.skia.org/search?issue=2253373002

Review-Url: https://codereview.chromium.org/2253373002
diff --git a/gn/gypi_to_gn.py b/gn/gypi_to_gn.py
old mode 100644
new mode 100755
index 0800708..c4f31ae
--- a/gn/gypi_to_gn.py
+++ b/gn/gypi_to_gn.py
@@ -1,3 +1,5 @@
+#!/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.