Turn on job-based sweeping

Embedders that don't use a v8::Platform yet will have to do so.

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

Review URL: https://codereview.chromium.org/443043002

git-svn-id: https://v8.googlecode.com/svn/branches/bleeding_edge@23562 ce2b1a6d-e550-0410-aec6-3dcde31c8c00