[Sheriff] Mark test-debug/* as flaky on Mac.

BUG=v8:3125
LOG=n
TBR=verwaest@chromium.org

Review URL: https://codereview.chromium.org/135183015

git-svn-id: https://v8.googlecode.com/svn/branches/bleeding_edge@18920 ce2b1a6d-e550-0410-aec6-3dcde31c8c00