Gardening: mark virtual/softwarecompositing as Timeout on Mac

BUG=377145
TBR=kbr

Review URL: https://codereview.chromium.org/293373002

git-svn-id: svn://svn.chromium.org/blink/trunk@174755 bbb929c8-8fbe-4397-9dbb-9b2b20218538