This allows to avoid conflicts with libraries in users' PATH

R=pgervais@chromium.org, tandrii@chromium.org

Review URL: https://codereview.chromium.org/1161803004

git-svn-id: svn://svn.chromium.org/chrome/trunk/tools/depot_tools@295497 0039d316-1c4b-4281-b951-d872f2087c98