I want to run Python unit tests as part of my presubmit, and those unit tests require PYTHONPATH to be set the way it would be set when the modules they test are running.

Review URL: https://codereview.chromium.org/250693002

git-svn-id: svn://svn.chromium.org/chrome/trunk/tools/depot_tools@265998 0039d316-1c4b-4281-b951-d872f2087c98