Search for buildtools in the gclient root, if not in the main repo.

BUG=https://code.google.com/p/chromium/issues/detail?id=430602
R=bradnelson@google.com, dpranke@chromium.org

Review URL: https://codereview.chromium.org/718213002

git-svn-id: http://src.chromium.org/chrome/trunk/tools/depot_tools@292912 4ff67af0-8c30-449e-8e8b-ad334ec8d88c