So that gsutil.py config works.

I would've preferred the execv solution, but apparently that didn't work on Windows :(

BUG=451551

Review URL: https://codereview.chromium.org/870093003

git-svn-id: svn://svn.chromium.org/chrome/trunk/tools/depot_tools@293790 0039d316-1c4b-4281-b951-d872f2087c98