tools/gn: add python shebang directive to roll_gn script

This way we can execute it from the command line like the following:

$ ./tools/gn/bin/roll_gn.py

Otherwise, on Linux you have to call python directly, like:

$ python ./tools/gn/bin/roll_gn.py

BUG=None
TEST=see above
R=dpranke@chromium.org
TBR=brettw@chromium.org

Review URL: https://codereview.chromium.org/1458603010

Cr-Commit-Position: refs/heads/master@{#360620}

tools/gn/bin/roll_gn.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.