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

This reverts commit r23285 for breaking cctest/test-api/Regress2107.

TBR=ulan@chromium.org, hpayer@chromium.org, yangguo@chromium.org

Review URL: https://codereview.chromium.org/494203002

git-svn-id: https://v8.googlecode.com/svn/branches/bleeding_edge@23286 ce2b1a6d-e550-0410-aec6-3dcde31c8c00