Start incremental marking in idle time handler only if it is worthwhile.

BUG=
R=hpayer@chromium.org

Review URL: https://codereview.chromium.org/492263002

git-svn-id: https://v8.googlecode.com/svn/branches/bleeding_edge@23285 ce2b1a6d-e550-0410-aec6-3dcde31c8c00