The cros wrapper in depot_tools is a centralized wrapper for all cros
commands. Adding this to depot_tools allows developers to run the 'cros'
tool from anywhere inside a Chrome OS checkout.

BUG=none
TEST=Verify the cros wrapper works.


Review URL: https://chromiumcodereview.appspot.com/11600002

git-svn-id: svn://svn.chromium.org/chrome/trunk/tools/depot_tools@173491 0039d316-1c4b-4281-b951-d872f2087c98