This CL adds pylint (and its dependencies) to third_party. It plumbs them into presubmit_canned_checks, and exposes a command-line tool to access pylint.

BUG=
TEST=

Review URL: https://chromiumcodereview.appspot.com/10447014

git-svn-id: svn://svn.chromium.org/chrome/trunk/tools/depot_tools@143016 0039d316-1c4b-4281-b951-d872f2087c98