chromite_wrapper: Add support for gclient checkouts.
chromite_wrapper: Add support for gclient checkouts.

Chromite will be added to the Chrome checkout at third_party/chromite -
detect when the checkout is a gclient checkout, and look for chromite at
that path.

BUG=None
TEST=ran 'cbuildbot' in both a repo and gclient checkout.


Review URL: https://chromiumcodereview.appspot.com/12094111

git-svn-id: bd64dd6fa6f3f0ed0c0666d1018379882b742947@181563 4ff67af0-8c30-449e-8e8b-ad334ec8d88c

svn2github/chromium-depot-tools,svn2github/chromium-depot-tools,svn2github/chromium-depot-tools