boto config does the right thing by using the http_proxy env var to set the proxy,
and gsutil removes that var which isn't the behavior we want.
This should allow gsutil to respect the http_proxy env var.

BUG=318478

Review URL: https://codereview.chromium.org/68763013

git-svn-id: svn://svn.chromium.org/chrome/trunk/tools/depot_tools@235818 0039d316-1c4b-4281-b951-d872f2087c98