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=hinoka@chromium.org, tandrii@chromium.org

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