Gardening: Add Mac10.8 (retina) to needed rebase list for test

TBR=enne@chromium.org
BUG=366438

Review URL: https://codereview.chromium.org/258793008

git-svn-id: svn://svn.chromium.org/blink/trunk@172874 bbb929c8-8fbe-4397-9dbb-9b2b20218538