Add the "gsutil" recipe module to depot_tools. This module is copied
from "tools/build"'s recipe module set at:
d6205ab0c0e8e1920f4af1617b5b3767cd944492.

Two Chromium-specific functions have been removed:
- download_with_polling
- download_latest_file

It also adds a function, "gsutil_py_path", which mimics the function in
the "depot_tools" recipe module. Ultimately, the latter will be phased
out in favor of the module-specific function.

BUG=chromium:662895
TEST=expectations

Change-Id: Ie049d637593b3f02b9ff029fbd20fd3f7170faa6
Reviewed-on: https://chromium-review.googlesource.com/411970
Commit-Queue: Daniel Jacques <dnj@chromium.org>
Reviewed-by: Robbie Iannucci <iannucci@chromium.org>