[download_from_google_storage] Use vpython to invoke gsutil.py.
[download_from_google_storage] Use vpython to invoke gsutil.py.

This will allow all uses of this class to correctly move into
gsutil's personal vpython environment. In particular, the
get_toolchain_if_necessary script is pretty sneaky, and adds
depot_tools to sys.path to import and use this class.  This
means that if get_toolchain_if_necessary is invoked in a vanilla
vpython environment (or an environment otherwise incompatible with
gsutil.py), it's use of this class will fail.

R=2190fe9a39b4920ec7bd59c848ec5cccd163056f@chromium.org, tandrii@chromium.org

Bug: 795146
Change-Id: I87309a8b878b465cda7f53db0a41f2f9e6da6f50
Reviewed-on: https://chromium-review.googlesource.com/849663
Reviewed-by: Ryan Tseng <2190fe9a39b4920ec7bd59c848ec5cccd163056f@chromium.org>
Commit-Queue: Robbie Iannucci <40f3d43a28ebae3cb819288542e1c84d73d962d5@chromium.org>

CoherentLabs/depot_tools,CoherentLabs/depot_tools