Add layout dbg trybot to default list.

BUG=353487
LOG=n
R=jkummerow@chromium.org

Review URL: https://codereview.chromium.org/281013003

git-svn-id: https://v8.googlecode.com/svn/branches/bleeding_edge@21302 ce2b1a6d-e550-0410-aec6-3dcde31c8c00