Switch download_from_google_storage to py3

Bug: 1199969
Change-Id: I5dcc5fc631cc417fd295d753171f3696f1c7ad0d
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/tools/depot_tools/+/2837688Reviewed-by: Linkun Chen <lkchen@chromium.org>
Reviewed-by: Josip Sokcevic <sokcevic@google.com>
Auto-Submit: Linkun Chen <lkchen@chromium.org>
Commit-Queue: Josip Sokcevic <sokcevic@google.com>

download_from_google_storage

@@ -5,4 +5,4 @@
 
 base_dir=$(dirname "$0")
 
-PYTHONDONTWRITEBYTECODE=1 exec python "$base_dir/download_from_google_storage.py" "$@"
+PYTHONDONTWRITEBYTECODE=1 exec python3 "$base_dir/download_from_google_storage.py" "$@"

download_from_google_storage.py

-#!/usr/bin/env python
+#!/usr/bin/env python3
 # Copyright (c) 2012 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.