Shell PATH was growing quite long in some cases.

R=gab@chromium.org, iannucci@chromium.org, johnw@chromium.org, maruel@chromium.org

Review URL: https://codereview.chromium.org/180303003

git-svn-id: svn://svn.chromium.org/chrome/trunk/tools/depot_tools@253347 0039d316-1c4b-4281-b951-d872f2087c98