Introduce --job-based-sweeping flag and use individual jobs for sweeping if set

BUG=v8:3104
R=hpayer@chromium.org
LOG=y

Review URL: https://codereview.chromium.org/143803007

git-svn-id: https://v8.googlecode.com/svn/branches/bleeding_edge@19357 ce2b1a6d-e550-0410-aec6-3dcde31c8c00