Add a DEPS file for Mojo

This CL adds a simple DEPS file for Mojo. It's better to add DEPS files early
before dependencies creep in.

R=aa@chromium.org

Review URL: https://codereview.chromium.org/27768002

git-svn-id: svn://svn.chromium.org/chrome/trunk/src@229231 0039d316-1c4b-4281-b951-d872f2087c98
SVN changes up to revision 229231