Shrink new space in idle notification.

BUG=chromium:424423
LOG=Y
R=jochen@chromium.org

Review URL: https://codereview.chromium.org/662543008

git-svn-id: https://v8.googlecode.com/svn/branches/bleeding_edge@24851 ce2b1a6d-e550-0410-aec6-3dcde31c8c00