Set execute permissions on sky_server after download.

BUG=
R=abarth@chromium.org

Review URL: https://codereview.chromium.org/886543004

sky/tools/download_sky_server

diff --git a/sky/tools/download_sky_server b/sky/tools/download_sky_server
index b29f6a3..c3a95a1 100755
--- a/sky/tools/download_sky_server
+++ b/sky/tools/download_sky_server
@@ -3,6 +3,8 @@
 # Use of this source code is governed by a BSD-style license that can be
 # found in the LICENSE file.
 
+import os
+import stat
 import subprocess
 
 from webkitpy.common.system import filesystem
@@ -10,11 +12,13 @@
 
 finder = WebKitFinder(filesystem.FileSystem())
 
+output_file_ = finder.path_from_chromium_base('out', 'downloads', 'sky_server')
 subprocess.call([
     'download_from_google_storage',
     '--no_resume',
     '--no_auth',
     '--bucket', 'mojo',
     '--sha1_file', finder.path_from_chromium_base('sky', 'tools', 'skygo', 'sky_server.sha1'),
-    '--output', finder.path_from_chromium_base('out', 'downloads', 'sky_server')
+    '--output', output_file_
 ])
+os.chmod(output_file_, os.stat(output_file_).st_mode | stat.S_IEXEC)