Add dependency on buildtools repo

Currently, this adds checkdeps, in the future, this will be the place
where clang-format and gn lives.

BUG=none
R=machenbach@chromium.org
LOG=n

Review URL: https://codereview.chromium.org/308353002

git-svn-id: https://v8.googlecode.com/svn/branches/bleeding_edge@21636 ce2b1a6d-e550-0410-aec6-3dcde31c8c00